Optical junction apparatus and methods employing optical power transverse-transfer

ABSTRACT

A method comprises: (i) forming a first optical waveguide on a first substrate; (ii) forming a second, structurally discrete optical waveguide on a structurally discrete second substrate; (iii) assembling the second substrate or second optical waveguide with the first substrate or first optical waveguide so that the first and second optical waveguides are positioned between the first and second substrates and are relatively positioned for transferring the optical signal therebetween via optical transverse coupling; and (iv) arranging the first or second optical waveguide for transferring the optical signal therebetween via substantially adiabatic optical transverse coupling with the first and second waveguides so positioned.

This application is a continuation of U.S. non-provisional applicationSer. No. 11/618,643 filed Dec. 29, 2006 (now U.S. Pat. No. 7,783,146),which is in turn a continuation of U.S. non-provisional application Ser.No. 11/333,933 filed Jan. 17, 2006 (now U.S. Pat. No. 7,158,702), whichis in turn a continuation of U.S. non-provisional application Ser. No.11/138,841 filed May 25, 2005 (now U.S. Pat. No. 7,050,681), which is inturn a divisional of 10/187,030 filed Jun. 28, 2002 (now U.S. Pat. No.6,987,913), which in turn claims benefit of U.S. provisional App. No.60/334,705 filed Oct. 30, 2001 and U.S. provisional App. No. 60/360,261filed Feb. 27, 2002. Each of said non-provisional and provisionalapplications is hereby incorporated by reference as if fully set forthherein.

BACKGROUND

The field of the present invention relates to opticaltelecommunications. In particular, optical junction apparatus andmethods are described herein employing transverse-transfer of opticalpower between assembled optical components.

This application is related to subject matter disclosed in:

A1) U.S. provisional Application No. 60/257,218 entitled “Waveguides andresonators for integrated optical devices and methods of fabrication anduse thereof” filed Dec. 21, 2000 in the name of Oskar J. Painter, saidprovisional application being hereby incorporated by reference in itsentirety as if fully set forth herein;

A2) U.S. provisional Application No. 60/301,519 entitled“Waveguide-fiber Mach-Zender interferometer and methods of fabricationand use thereof” filed Jun. 27, 2001 in the names of Oskar J. Painter,David W. Vernooy, and Kerry J. Vahala, said provisional applicationbeing hereby incorporated by reference in its entirety as if fully setforth herein;

A3) U.S. provisional Application No. 60/322,272 entitled“Fiber-optic-taper probe for characterizingtransversely-optically-coupled waveguides and resonators” filed Sep. 13,2001 in the name of David W. Vernooy, said provisional application beinghereby incorporated by reference in its entirety as if fully set forthherein;

A4) U.S. Pat. No. 5,032,219 entitled “Method for improving the planarityof etched mirror facets” issued Jul. 16, 1991 in the names of Peter. L.Buchman, Peter Vettiger, Otto Voegeli, and David J. Webb, said patentbeing hereby incorporated by reference in its entirety as if fully setforth herein;

A5) U.S. Pat. No. 5,103,493 entitled “Improved planar etched mirrorfacets” issued Apr. 7, 1992 in the names of Peter. L. Buchman, PeterVettiger, Otto Voegeli, and David J. Webb, said patent being herebyincorporated by reference in its entirety as if fully set forth herein;

A6) U.S. Pat. No. 5,177,031 entitled “Method of passivating etchedmirror facets of semiconductor laser diodes” issued Jan. 5, 1993 in thenames of Peter. L. Buchman, David J. Webb, and Peter Vettiger, saidpatent being hereby incorporated by reference in its entirety as iffully set forth herein;

A7) U.S. Pat. No. 5,259,049 entitled “Self-aligned optical waveguide tolaser structure and method of making the same” issued Nov. 2, 1993 inthe names of Gian-Luca Bona, Fritz Gfeller, Heinz Jaeckel, and David J.Webb, said patent being hereby incorporated by reference in its entiretyas if fully set forth herein;

A8) U.S. provisional App. No. 60/334,705 entitled “Integratedend-coupled transverse-optical-coupling apparatus and methods” filedOct. 30, 2001 in the names of Henry A. Blauvelt, Kerry J. Vahala, PeterC. Sercel, Oskar J. Painter, and Guido Hunziker, said application beinghereby incorporated by reference in its entirety as if fully set forthherein;

A9) U.S. provisional App. No. 60/333,236 entitled “Alignment apparatusand methods for transverse optical coupling” filed Nov. 23, 2001 in thenames of Charles I. Grosjean, Guido Hunziker, Paul M. Bridger, and OskarJ. Painter, said application being hereby incorporated by reference inits entirety as if fully set forth herein;

A10) U.S. non-provisional application Ser. No. 10/037,966 (now U.S. Pat.No. 6,839,491) entitled “Multi-layer dispersion-engineered waveguidesand resonators” filed Dec. 21, 2001 in the names of Oskar J. Painter,David W. Vernooy, and Kerry J. Vahala, said application being herebyincorporated by reference in its entirety as if fully set forth herein;and

A11) U.S. provisional App. No. 60/360,261 entitled“Alignment-insensitive optical junction apparatus and methods employingadiabatic optical power transfer” filed Feb. 27, 2002 in the names ofHenry A. Blauvelt, Kerry J. Vahala, David W. Vernooy, and Joel S.Paslaski, said provisional application being hereby incorporated byreference as if fully set forth herein.

This application is also related to subject matter disclosed in thefollowing publications, each of said publications being herebyincorporated by reference in its entirety as if fully set forth herein:

-   P1) Y. P. Li and C. H. Henry, Silicon Optical Bench Waveguide    Technology, in Optical Fiber Telecommunications, IIIb, I. P. Kaminow    and T. L. Koch eds., Academic Press, 1997;-   P2) T. Ramadan, R. Scarmozzino, and R Osgood “Adiabatic Couplers:    Design Rules and Optimization” IEEE J. Lightwave Tech., v16, No. 2,    pp 277-283, (1998);-   P3) D. G. Dalgoutte, R. B. Smith, G. Achutaramayya, and J. H.    Harris, “Externally mounted fibers for integrated optics    interconnections”, Appl. Optics Vol. 14, No. 8, pp 1860-1865 (1975);    and-   P4) Y. Shani, C. H. Henry, R. C. Kistler, R. F. Kazarinov, and K. J.    Orlowsky, “Integrated optic adiabatic devices on silicon”, IEEE J.    Quant. Elec., Vol. 27, No. 3, pp 556-566 (1991).

A fundamental problem in the field of optical telecommunications isattaining efficient and cost-effective transfer of optical signal powerbetween assembled optical components. One particularly significantexample is achieving optical signal power transfer between an active orpassive optical device and a low-loss transmission optical waveguide,including optical fibers and/or planar waveguide circuits. Examples ofactive optical devices may include but are not limited to semiconductorlasers, electro-absorption modulators, electro-absorption modulatedlasers, electro-optic modulators, semiconductor optical amplifiers,photodiodes or other photodetectors, N×N optical switches, and so forth.Examples of passive devices may include but are not limited towavelength division multiplexers/de-multiplexers, wavelength divisionslicers/interleavers, wavelength division add/drop filters, otheroptical filters, splitters/combiners, interferometers, phase shifters,dispersion compensators, fixed or variable optical attenuators, and soforth. Such optical devices often involve generation of, interactionwith, and/or manipulation of optical modes that are typically small(particularly in semiconductor-based devices), often on the order ofjust a few microns across and sometimes less than 1 micron high. Thisinteracting mode size is typically much smaller than an optical modesize supported by a single-mode optical fiber or a planar lightwavecircuit (generally about ten microns across). End-coupling of an opticalfiber or planar waveguide circuit to an optical device is thereforeoften inefficient (around 5-15%) due to spatial mode mismatch, yieldingdevices having undesirably large insertion losses. Prior artmethodologies exist for achieving higher end-coupling efficiencies, butthese require expensive components for achieving better mode-matching(aspheric lenses and the like), and also require high-precision activealignment of the optical components and the optical device (requiredtolerances may be as small as 0.1 μm, and must typically be achieved onan individual device basis).

Prior art methodologies exist for low-cost end-coupled optical assembly(such as methodologies based on silicon optical bench technologies, forexample). However, these low-cost solutions generally suffer from lowoptical power transfer efficiency between an optical device and anoptical fiber or other waveguide, for the reasons set forth hereinabove.

Optical power transfer by end-coupling (equivalently, end-fire couplingor end-transfer) is characterized by positioning of the opticalcomponents in an end-to-end geometry substantially along the directionof propagation of the optical signal power to be transferred. At theoptical junction thus formed, optical power propagates out through anend-face of one optical component and in through an end-face of anotheroptical component. Alternatively, optical power transfer may be achievedby so-called transverse-coupling (equivalently, transverse-transfer), inwhich the optical components are positioned in a side-by-side geometryrelative to the direction of propagation of the optical signal power. Atthe optical junction formed by transverse-coupling, there is typicallyat least one segment of the junction with optical power propagatingalong both components simultaneously.

Efficient end-transfer between optical components requires that opticalmodes in the respective components be substantially spatial-modematched. Transverse-transfer of optical power between an optical deviceand a transmission optical waveguide provides an alternative toend-transfer for transferring optical signal power between an opticaldevice and a transmission waveguide (through a taper segment of anoptical fiber or through a suitably adapted portion of a planarwaveguide, for example). In particular, the requirement for spatial-modematching is eliminated; transverse-transfer of optical power may beachieved between optical modes of differing spatial-mode size and/orshape.

Transverse-transfer (also referred to as transverse coupling, transverseoptical coupling, evanescent optical coupling, evanescent coupling,directional optical coupling, directional coupling) is discussed atlength in several of the prior patent applications cited hereinabove,and the entire discussion need not be repeated herein.Transverse-transfer may be readily described in terms of optical modescharacteristic of the separate optical waveguides (or other opticalcomponents) transitioning to the optical modes characteristic of acoupled-waveguide optical system. These latter modes are referred toherein as the “system modes” or “coupled-system modes”, while the formermodes are referred to herein as the “isolated modes” or“isolated-waveguide modes”. Efficient transfer of optical signal powerbetween optical waveguides by transverse-coupling may be achieved in oneof several operating regimes. Two such regimes discussed herein areso-called mode-interference coupling and so-called adiabatic opticalpower transfer.

In so-called mode-interference coupling (described in several of theabove-cited references, particularly A8 and A10, and referred to thereinsimply as transverse optical coupling), optical signal power entering ajunction region from one waveguide is divided between two guided systemmodes. Ideally, this transition into the junction region is configuredso that the isolated mode is very nearly a linear superposition of thetwo lowest order system modes. This condition results in minimal powerloss to higher order system modes (and/or radiation modes) as opticalsignal power enters the junction region. The two system modes propagatethrough the junction region along the waveguides with differingpropagation constants (designated as β+ and β− for the two lowest-ordersystem modes). Upon reaching the end of the junction region, opticalsignal power is divided into the two waveguides according to therelative phase of the two system modes. Once again, to minimize loss tohigher-order and/or radiative modes, the isolated modes shouldsubstantially resemble linear superpositions of the two system modes.Since this is typically the case in practical devices, and presents areasonable approximation even when it is not the case, it is usuallypossible to describe the characteristics of the junction region in termsof properties of the isolated modes, and such a description shall beused hereinafter. In particular, the degree of optical signal powertransfer via mode-interference coupling is determined by the degree oftransverse overlap between the isolated-waveguide modes, (characterizedby a coupling coefficient κ), by the propagation distance over which themodes overlap (i.e., junction region length or interaction length L),and by the degree of modal index mismatch (characterized by Δβ=β₁−β₂,the β's being the propagation constants for the respectiveisolated-waveguide modes). In mode-interference coupling, κ, β₁, β₂ aretypically assumed to remain substantially constant over the length L ofthe junction region. Transfer of optical power between themode-interference-coupled waveguides is given by (neglecting the effectsof optical losses):

$\begin{matrix}{\frac{{{E_{2}(L)}}^{2}}{{{E_{1}(0)}}^{2}} = {\frac{{\kappa }^{2}}{q^{2}}{\sin^{2}\left( \begin{matrix}q & \left. L \right)\end{matrix} \right.}}} \\{q^{2} = {{\kappa }^{2} + {\frac{1}{4}{{\Delta\beta}^{2}.}}}}\end{matrix}$where the following definitions apply:

E_(1,2)(z) amplitudes of the coupled fields;

β_(1,2) propagation constants of the coupled fields;

κ coupling amplitude resulting from spatial overlap of the fields;

z longitudinal propagation distance coordinate

An incident field of amplitude E₁ that is spatially confined to a firstoptical waveguide before the junction region will transfer to the otheroptical element with a resultant field amplitude E₂(L) at z=L (where wedefine z=0 as the start of the junction region and z=L as the end of thejunction region). Optical power transfer as a function of the junctionregion length L is therefore oscillatory with a characteristic period or“beat length” that depends on κ and Δβ. This may be thought of as amanifestation of the interference between the system modes excitedwithin the junction region, both of which carry optical signal power.Greater coupling amplitude κ and/or greater modal-index mismatch Δβ willreduce the beat length. The absolute magnitude of the oscillatory powertransfer decreases with increasing modal-index mismatch, withsubstantially complete transfer of optical power back and forth betweenthe optical elements when Δβ is near zero. A particular degree ofoptical power transfer from one waveguide to the other may be achievedby configuring the junction region with the length L to achieve thedesired transfer fraction for a given Δβ and κ.

To understand the distinction between mode interference coupling andadiabatic power transfer, it is first necessary to understand themeaning of the adiabatic condition within the general context of anoptical waveguide. Two examples are presented for illustration. Considerfirst a single mode waveguide that is tapered over some segment of itslength so as to modify both the transverse extent and the propagationconstant of the guided mode. Tapering of a waveguide supporting even asingle mode induces coupling to radiation modes. However, provided thatthe tapering is sufficiently gradual so that this radiative loss is weak(i.e., adiabatic tapering), it still makes sense to consider the opticalpower traversing the tapered waveguide as representing a single mode,albeit one whose properties have a longitudinal position dependence(i.e. z-dependence) as it traverses the tapered waveguide segment.Provided the adiabatic condition is satisfied (i.e., tapering is slowenough to render coupling to other modes minimal or below anoperationally acceptable level), it is possible to describe the modeusing longitudinally varying quantities such as a z-dependentpropagation “constant” β(z).

As a second example, the properties of a waveguide could be varied alongthe longitudinal propagation direction so that the waveguide at oneposition supports a single transverse mode while at another positionsupports two or more transverse modes. In this example, adiabaticvariation of the waveguide properties would result in negligible (oroperationally acceptable) coupling to these other modes so that onceagain it is possible to think of the single “mode” as being preserved asit propagates along the waveguide, albeit as a mode whose propertiessuch as its propagation “constant” β and/or its transverse spatialprofile acquire a dependence on longitudinal position z along thewaveguide.

This approximate way of considering optical modes subject to anadiabatic variation along the longitudinal or propagation direction isan important concept for understanding the operation of adiabatic powertransfer devices. It is important to note that the term “mode” acquiresa slightly more general meaning in the context of waveguides andjunctions that satisfy an adiabatic condition. In particular, to theextent that coupling to other modes is minimal or remains at or belowsome operationally acceptable level, the terms “mode” and/or “opticalmode” shall be used herein even if spatial, temporal, polarization,and/or other properties might evolve as the mode propagates along awaveguide whose properties vary longitudinally in an adiabatic fashion.This more general interpretation of modes is distinct from the moreconventional use of the term “mode” which may typically implypreservation of certain modal properties, such as propagation constantβ, transverse spatial profile, polarization state, and so on, as themode propagates along a substantially longitudinally invariantwaveguide.

For adiabatic optical power transfer, two isolated modes a₁(z) and a₂(z)characteristic of the isolated waveguides begin to experience weakcoupling as they enter the junction region. Under the adiabaticcondition this weak coupling may be characterized by a couplingcoefficient κ(z) and modal-index mismatch Δβ(z)=β₁(z)−β₂(z). Theresulting system modes will substantially resemble the superpositionmodes a+(z) and a−(z) of the coupled-waveguide system given by

$a_{\pm} = {{{\frac{\lambda_{\pm}}{\sqrt{\kappa^{2} + \lambda_{\pm}^{2}}}a_{1}} + {\frac{\kappa}{\sqrt{\kappa^{2} + \lambda_{\pm}^{2}}}a_{2}\mspace{14mu}{where}\mspace{14mu}\lambda_{\pm}}} = {\left( \frac{\Delta\beta}{2} \right) \pm \sqrt{\kappa^{2} + \left( \frac{\Delta\beta}{2} \right)^{2}}}}$where all quantities are z-dependent. For purposes of the presentdiscussion, the terms “superposition modes” and “system modes” may beused interchangeably, even though the system modes may not resemble thesuperposition modes throughout the junction region. At the beginning ofthe junction region (i.e., z=0), superposition mode a₊ preferablyclosely resembles only one of the isolated-waveguide modes a₁ or a₂,while mode a_ resembles the other. For example, in the limit of|Δβ|>>|κ| (i.e., strongly modal-index mis-matched),

$\alpha_{+} \approx {\alpha_{1} + {\frac{\kappa}{\Delta\beta}a_{2}}} \approx a_{1}$and

${a_{-} \approx {a_{2} - {\frac{\kappa}{\Delta\beta}a_{1}}} \approx a_{2}},$meaning each superposition mode is predominantly associated with asingle isolated-waveguide mode in this limit (i.e., a₊

a₁ and a_

a₂). For adiabatic optical coupling, preferably |Δβ|>>|κ| for theisolated-waveguide modes at z=0. Under this input termination condition,the superposition modes (and hence also the system modes) substantiallyresemble the isolated-waveguide modes, and optical signal power enteringthe junction region in a first waveguide is transferred predominantly(even exclusively) into the corresponding system mode. The junctionregion is configured so that |Δβ| (for the isolated-waveguide modes)initially decreases along the junction region. The coefficient κ mayalso vary along the junction region, preferably reaching a maximumabsolute value within the junction region. As evident from the equationsdefining the superposition modes given above, the variation of Δβ and/orκ results in evolution of the superposition modes (more precisely, thesystem modes) along the length of the junction region. As describedabove, the adiabatic condition requires that the variation of Δβ and/orκ must be sufficiently gradual so that transfer of optical power betweensystem modes and/or between a system mode and other optical modes(guided or otherwise) remains at or below some operationally acceptablelevel. This criterion is equivalent to the adiabatic condition describedin reference P2. In particular, any change in waveguide spacing,transverse dimensions, modal and/or material index, or other properties(before, within, and/or after the junction region) must be sufficientlygradual so as to minimize or reduce to an operationally acceptable leveloptical power transfer into undesirable modes of the coupled-waveguidesystem.

The “approach regions” of the joined waveguides (i.e., the regionsdirectly before and after the junction region; may also be referred toas input and output regions) should preferably be adapted to satisfy theadiabatic condition. The waveguides to be joined may typically approacheach other at a fairly shallow angle in order to minimize undesirableoptical power transfer or optical loss that might result from an abruptapproach. Alternatively, one waveguide may arise from a narrow tip andincrease in height and/or width along the length of the other waveguidebefore reaching its full transverse dimensions. This gradual“appearance” of optical material may be made sufficiently gradual so asto satisfy/maintain the adiabatic condition. Similarly, after thejunction region, the waveguides may move apart at a shallow angle, orone waveguide may decrease in transverse dimension(s) until itterminates in a narrow tip. The relative lengths of the approach regionsand the junction region will typically depend on the strength of theinteraction between the joined waveguides. For strong interactionbetween the waveguides in the junction region, the junction region mightbe relatively short, while very gradual approach and separation of thewaveguides (and correspondingly longer approach regions) may be requiredto maintain an adiabatic condition. On the other hand, weakerinteraction between the waveguides in the junction region requires arelatively longer junction region to achieve a given level of opticalpower transfer, but shorter approach regions may be used whilenevertheless substantially avoiding undesirable optical power transferto other optical modes. For a given waveguide type/geometry, it shouldbe possible to achieve a desired level of optical power transfer betweenthe waveguides with undesirable optical coupling maintained below someoperationally acceptable level, while minimizing the overall length ofthe adiabatic optical power transfer device. If a higher level ofundesirable optical coupling is tolerable (i.e., operationallyacceptable) in a given device, shorter approach regions may be employedin order to reduce overall device size. It should be noted that theapproach regions and junction region may not be clearly demarcated, butinstead may gradually transition from one to the next. Such gradualtransitions are typically necessary in order to satisfy the adiabaticcondition.

For achieving substantially complete transfer of optical power betweenthe waveguides, Δβ preferably reaches zero and changes sign at somepoint within the junction region, after which |Δβ| increases along thejunction region. At the end of a sufficiently long junction region(i.e., |Δβ|>>|κ| at z=L; output termination condition), the system modecarrying the optical power has evolved to substantially resemble theisolated-waveguide mode of the second waveguide, and the optical powerleaves the coupling region in the second waveguide. The first waveguidemay or may not terminate at the end of the junction region or shortlythereafter, provided that such termination satisfies the adiabaticcondition. Likewise, the second waveguide may only appear at thebeginning of the junction region or shortly before, provided that suchappearance satisfies the adiabatic condition.

It is important to note that adiabatic transfer of optical power fromthe first waveguide to the second waveguide is accomplished without theuse of “mode coupling.” In particular, optical power leaves the junctionregion on the second waveguide carried by the “same” system mode as thesystem mode that carried the optical signal power entering the junctionregion on the first waveguide. This occurs since the adiabatic conditiondictates that only negligible (or at most operationally acceptable)optical power transfer to other modes has occurred during the transferof optical power between the waveguides (i.e., the system mode has beenpreserved by the adiabatic properties of the junction, even though itsphysical appearance has evolved in transit through the junction region).This behavior is quite distinct from the behavior of mode-interferencecoupling, which relies upon optical power being carried through thejunction region by multiple system modes (usually two) to achieveoptical power transfer.

In order to achieve division of optical power leaving the junctionregion between the two adiabatic-coupled waveguides (having entered thejunction region through only one of them), the junction region may beconfigured so that at z=L the system mode substantially resembles asuperposition mode that includes substantial components of bothisolated-waveguide modes. Under these conditions optical power in thesystem mode will be divided into the two isolated-waveguide modes andleave the junction region in both waveguides. For example, a desiredfraction of optical power transfer of about 50% (i.e., about 3 dB) maybe desirable for implementing an interferometric device. An adiabaticoptical power transfer junction may be employed having |Δβ| decreasingto about zero and then remaining near zero over the remaining length Lof the junction region. The resulting system modes may havesubstantially equally weighted components substantially corresponding toeach of the isolated-waveguide modes at the end of the junction region,resulting in substantially equal fractions of optical power leaving thejunction region in each waveguide. Other fractions of optical powertransfer may be implemented by employing adiabatic transverse opticalpower transfer as required for a specific device.

In contrast to the behavior of mode-interference-coupled waveguides, inwhich optical power transfer oscillates as a function of the junctionlength L, the fraction of power transfer for adiabatic optical powertransfer is a substantially monotonic function of the distance L,typically closely approaching an asymptotic value after a certainminimum distance (which depends on κ and Δβ) and then remainingsubstantially unchanged with additional junction region length. Thisfundamental difference in behavior has a profound influence on thefabrication/assembly/alignment tolerances required for producingtransverse-coupled optical components. Briefly, variations in κ and/orΔβ may affect the minimum junction region length required to achieve adesired level of optical power transfer between waveguides, but do nottypically affect the asymptotic fraction of optical power transferred.As long as the junction region of an assembled device is longer than thelargest such minimum junction region length likely to arise due tofabrication/assembly/alignment variations, then the fraction of opticalpower transfer in the assembled device will remain substantiallyunaffected. This is discussed in more detail below, and is an importantfeature of the present invention.

Frequently the desired objective of an optical junction device is toeffect a specific degree of optical power transfer from one opticalcomponent to another optical component assembled therewith. Achieving aspecifically-desired degree of optical power transfer usingmode-interference coupling requires design, fabrication, and assembly oftransverse-optical-coupled elements having κ, Δβ, and L kept withintight tolerances (although not as tight as tolerances required forend-coupling, as discussed in reference A8). Variation in relativepositioning of the optical elements (affecting κ and possibly also Δβ)causes variation in the “beat length”, and hence the degree of opticalpower transfer for a given junction region length L (which may typicallyrange between several tens to about 100 μm). For example, a fiber-optictaper segment (diameter 2-3 μm) mode-interference-coupled to a topsurface of a dielectric waveguide on a substrate (3-5 μm wide) mayrequire positioning within ±0.5 μm accuracy horizontally and ±20 nmaccuracy vertically to keep nominally complete (100%) optical powertransfer above the 90% level (0.5 dB level). Such tolerances may bedifficult, expensive, and/or time-consuming to achieve, and may reducedevice yield, particularly in a mass-production environment. Similarly,the mode-interference-coupled elements must be designed and fabricatedsufficiently accurately to yield sufficiently accurate Δβ and κ.Variation in Δβ gives rise to variation in the beat length as well asthe maximum degree of optical power transfer that may be achieved.Further discussion of mode-interference coupling, and optical couplingin general, may be found in Fundamentals of Photonics by B. E. A. Salehand M. C. Teich (Wiley, New York, 1991), hereby incorporated byreference in its entirety as if fully set forth herein. Particularattention is called to Chapter 7 and Chapter 18.

Adiabatic optical power transfer may be exploited to further relaxmanufacturing tolerances for assembled optical components and devicesrelative to mode-interference coupling. For example, to achievesubstantially complete transfer of optical power from one waveguide toanother using adiabatic optical power transfer, the length of thejunction region should be made sufficiently long (typically severalhundred μm up to perhaps several mm) so that substantially completeoptical power transfer occurs for nearly all values of |κ| and |Δβ|likely to arise during fabrication and assembly of an optical device.Manufacturing variations in κ and Δβ would therefore have little or noeffect on the substantially complete transfer of optical power betweenwaveguides (in contrast to the situation with mode-interferencecoupling). For example, in the example given above of a fiber-optictaper segment (diameter 2-3 μm) coupled to a top surface of a dielectricwaveguide on a substrate (3-5 μm wide, with a modal index varying over ajunction region several hundred μm in length) may only require positionaccuracy within limits about 3 to 5 times larger than those required formode-interference coupling.

The techniques and configurations of adiabatic optical power transfermay therefore be exploited for constructing optical devices that includeinitially separate optical components subsequently assembled together,thereby providing apparatus and methods for transferring optical signalpower between optical components that overcome various drawbacksdescribed hereinabove. It is desirable to provide apparatus and methodsfor transferring optical signal power between waveguides joined by anadiabatic optical junction. It is desirable to implement apparatus andmethods for adiabatic optical power transfer wherein fabrication,assembly, and/or alignment tolerances are substantially relaxed relativeto end-coupling and mode-interference transverse-coupling. It isdesirable to enable passive alignment of the waveguides. It is desirableto provide at least one of the waveguides as an integrated opticalcomponent on a substrate. It is desirable to implement substantiallyadiabatic optical power transverse-transfer adiabatic apparatus andmethods that may be compatible with established optical devicetechnologies.

A fundamental problem in the field of fiber-optic telecommunications isefficient transfer of optical signal power between the optical fiber andthe optical devices for generating and/or manipulating the opticalsignal power. Transverse-transfer of optical power may be advantageouslyemployed to transfer optical power between an optical fiber and anoptical device through an intermediate external-transfer opticalwaveguide. It is desirable to provide apparatus and methods fortransferring optical signal power between an optical device on asubstrate and a transmission waveguide through an external-transferwaveguide optically integrated with the optical device on the substrate,wherein optical signal power is transferred between theexternal-transfer waveguide and the transmission waveguide by opticalpower transverse-transfer (adiabatic or otherwise). Optical power may betransferred between the device and the external-transfer waveguide byend-transfer or transverse-transfer (adiabatic or otherwise). Anexternal-transfer waveguide adapted for end-transfer with the opticaldevice may be substantially spatial-mode-matched therewith. Thetransmission optical waveguide may be the optical fiber (suitablyadapted for transverse-transfer) or may be a planar waveguide. Such aplanar transmission optical waveguide may more readily enable transferof optical signal power to/from the optical fiber. It is desirable toimplement optical power transfer via external-transfer waveguideapparatus and methods that may be compatible with established opticaldevice technologies. An external-transfer optical waveguide (adapted foroptical power transverse-transfer, adiabatic or otherwise, with atransmission waveguide) may be a component optically integrated with anoptical device, and may be provided using precision spatially selectivefabrication and processing techniques similar to those used to fabricateand process the optical device. Use of such fabrication techniquesthereby enables wafer-scale fabrication and precision alignment of manyexternal-transfer waveguide/device pairs in parallel on a singlesubstrate, thereby realizing significant economies of time and cost tomanufacture optical devices. It is desirable to enable and/or facilitatesubstantially simultaneous assembly/alignment of an optical device withtwo or more transmission waveguides.

SUMMARY

Certain aspects of the present invention overcome one or moreaforementioned drawbacks of the previous art and/or advance thestate-of-the-art of optical power transfer, and in addition may meet oneor more of the following objects:

-   -   To provide apparatus and methods for substantially adiabatic        optical power transverse-transfer between optical waveguides;    -   To provide mechanically separate optical waveguides adapted for        substantially adiabatic optical power transverse-transfer when        assembled;    -   To provide waveguides adapted for substantially adiabatic        optical power transverse-transfer therebetween with relaxed        fabrication and/or alignment tolerances for optical power        transfer;    -   To provide apparatus and methods for substantially adiabatic        optical power transverse-transfer between optical waveguides        wherein optical power transverse-transfer is less sensitive to        dimension(s) and/or relative position of the waveguides than is        the case for end-transfer and/or mode-interference-coupled        transverse-transfer;    -   To provide mechanically separate optical waveguides adapted for        substantially adiabatic optical power transverse-transfer when        aligned passively and assembled;    -   To provide apparatus and methods for substantially adiabatic        optical power transverse-transfer between waveguides wherein the        power transfer level remains substantially flat over a        substantial range of relative transverse positions of the        waveguides;    -   To provide apparatus and methods for substantially adiabatic        optical power transverse-transfer between waveguides wherein the        power transfer level remains within about 0.5 dB of its maximum        level over a range of waveguide transverse offsets, the range        being larger than about ±1.0 times a corresponding transverse        spatial mode size characteristic of the waveguides;    -   To provide at least one of the optical waveguides adapted for        substantially adiabatic optical power transverse-transfer as an        integrated optical component on a substrate;    -   To provide at least one of the optical waveguides adapted for        substantially adiabatic optical power transverse-transfer as a        planar waveguide on a substrate;    -   To provide substantially adiabatic optical power        transverse-transfer apparatus and methods that may be compatible        with established optical device technologies;    -   To provide apparatus and method for enabling and/or facilitating        transfer of optical signal power between an optical fiber and an        optical device on a substrate;    -   To provide apparatus and methods for transferring optical signal        power between an optical device and a transmission optical        waveguide through an external-transfer optical waveguide, the        external-transfer optical waveguide being optically integrated        with the optical device on a common substrate, the        external-transfer optical waveguide and the transmission optical        waveguide being adapted for optical power transverse-transfer        therebetween;    -   To provide a suitably adapted (for transverse-transfer) optical        fiber as the transmission optical waveguide;    -   To provide a suitably adapted (for transverse-transfer) planar        waveguide as the transmission optical waveguide;    -   To provide a suitably adapted (for transverse-transfer) planar        waveguide as the transmission optical waveguide, the planar        waveguide being further adapted for transferring optical power        to/from an optical fiber;    -   To provide the optical device and/or the external-transfer        optical waveguide adapted for substantially spatial-mode-matched        end-transfer of optical power therebetween;    -   To provide the optical device and/or the external-transfer        optical waveguide adapted for transverse-transfer of optical        power therebetween;    -   To provide the external-transfer optical waveguide and/or the        transmission optical waveguide adapted for substantially        adiabatic optical power transverse-transfer therebetween;    -   To provide the external-transfer optical waveguide and/or the        transmission optical waveguide adapted for        passively-modal-index-matched mode-interference-coupled optical        power transverse-transfer therebetween;    -   To provide the external-transfer optical waveguide and/or the        transmission optical waveguide adapted for        actively-modal-index-matched mode-interference-coupled optical        power transverse-transfer therebetween;    -   To provide the transmission optical waveguide and the integrated        optical device/external-transfer optical waveguide as        mechanically separate components adapted for optical power        transverse-transfer between the external-transfer optical        waveguide and the transmission optical waveguide when assembled;    -   To provide the transmission optical waveguide and the integrated        optical device/external-transfer optical waveguide as        mechanically separate components adapted for optical power        transverse-transfer between the external-transfer optical        waveguide and the transmission optical waveguide when passively        aligned and assembled;    -   To provide the optically integrated external-transfer waveguide        by applying to the optical device precision material processing        techniques such as lithography, deposition, masking, and/or        etching techniques, thereby enabling precision alignment of the        external-transfer optical waveguide and the optical device;    -   To provide external-transfer optical waveguide apparatus and        methods that may be compatible with established optical device        technologies;    -   To provide wafer-scale fabrication and precision alignment of        many optically integrated external-transfer waveguide/device        pairs in parallel on a single substrate;    -   To provide apparatus and methods enabling assembly/alignment of        multiple optical devices on a common planar waveguide substrate,        the devices each being adapted for optical power transfer with        one or more planar transmission waveguides on the planar        waveguide substrate;    -   To enable and/or facilitate substantially simultaneous        assembly/alignment of a waveguide with two or more other        transmission waveguides so as to establish optical power        transverse-transfer therebetween; and    -   To enable and/or facilitate substantially simultaneous        assembly/alignment of an optical device with two or more        waveguides so as to establish an optical junction between the        device and each waveguide.

One or more of the foregoing objects may be achieved in the presentinvention by an apparatus for transferring optical power between a firstoptical waveguide and a second optical waveguide, the first and secondwaveguides being initially mechanically separate and subsequentlyassembled to form an optical junction for optical power transfer. Theapparatus comprises: a first optical waveguide including an opticaljunction region; and a second optical waveguide including an opticaljunction region. The junction region of one or both of the waveguidesis/are adapted for substantially adiabatic optical power transfer,through variation of one or more optical properties along the lengththereof. Longitudinal variation of dimension(s) and/or opticalproperties of the first and second waveguides are sufficiently gradualso as to result in undesirable optical power transfer between the guidedsystem optical mode and other optical modes at or below an operationallyacceptable level. Assembly of the first and second waveguides serves toposition the respective optical junction regions thereof so as to enablesubstantially adiabatic optical power transfer between the waveguides.The nature of adiabatic optical power transfer results in substantiallyrelaxed alignment tolerances relative to end-transfer and/ormode-interference-coupled transverse-transfer. Passive alignment may beemployed to assemble the first and second waveguides for optical powertransfer. The junction regions may be adapted so as to providesubstantially complete transfer of optical power between the waveguides.

One or more of the foregoing objects may be achieved in the presentinvention by an apparatus for transferring optical power between anoptical device and a transmission optical waveguide, the apparatuscomprising: a) an optical device on a substrate; b) a transmissionoptical waveguide; and c) an external-transfer optical waveguideoptically integrated with the optical device, the external-transferwaveguide being adapted for transmitting optical power between theoptical device and the transmission optical waveguide. The opticaldevice and/or the external-transfer optical waveguide may preferably beadapted and positioned for end-transfer or transverse-transfer ofoptical power therebetween. The external-transfer waveguide and/or thetransmission optical waveguide may preferably be adapted fortransverse-transfer of optical power therebetween(mode-interference-coupled or adiabatic). The transmission opticalwaveguide is provided initially as a component mechanically separatefrom the optical device and external-transfer optical waveguide.Assembly of the transmission optical waveguide with the substrate,optical device, and/or external-transfer optical waveguide serves toposition the transmission optical waveguide and the external-transferoptical waveguide for transverse-transfer of optical power therebetween.An external-transfer waveguide optically integrated with an opticaldevice may be provided using wafer-scale fabrication and processingtechniques, thereby enabling wafer-scale fabrication and precisionalignment of many external-transfer waveguide/device pairs in parallelon a single substrate. The transmission optical waveguide may be asuitably adapted (for transverse-transfer with the external-transferoptical waveguide) optical fiber, or may preferably be a suitablyadapted (for transverse-transfer with the external-transfer opticalwaveguide) planar transmission optical waveguide. The planar waveguidemay be further adapted for transferring optical power to/from an opticalfiber, thereby enabling transfer of optical power between the device andthe fiber through the integrated external-transfer waveguide and theassembled planar transmission waveguide. The planar transmissionwaveguide may be part of a planar waveguide circuit.

Additional objects and advantages of the present invention may becomeapparent upon referring to the preferred and alternative embodiments ofthe present invention as illustrated in the drawings and described inthe following written description and/or claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C are top, side, and end views, respectively, ofwaveguides adapted and assembled for substantially adiabatic opticalpower transfer according to the present invention.

FIGS. 2A, 2B, and 2C are top, side, and end views, respectively, ofwaveguides adapted and assembled for substantially adiabatic opticalpower transfer according to the present invention.

FIGS. 3A, 3B, and 3C are top, side, and end views, respectively, ofwaveguides adapted and assembled for substantially adiabatic opticalpower transfer according to the present invention.

FIGS. 4A, 4B, and 4C are top, side, and end views, respectively, ofwaveguides adapted and assembled for substantially adiabatic opticalpower transfer according to the present invention.

FIGS. 5A, 5B, and 5C are top, side, and end views, respectively, ofwaveguides adapted and assembled for substantially adiabatic opticalpower transfer according to the present invention.

FIGS. 6A and 6B are top and side views, respectively, of waveguidesadapted and assembled for substantially adiabatic optical power transferaccording to the present invention.

FIGS. 7A and 7B are top and end views, respectively, of waveguidesadapted and assembled for substantially adiabatic optical power transferaccording to the present invention.

FIGS. 8A and 8B are side and end views, respectively, of waveguidesadapted and assembled for substantially adiabatic optical power transferaccording to the present invention.

FIGS. 9A, 9B, and 9C are top, side, and end views, respectively, of awaveguide and fiber-optic adapted and assembled for substantiallyadiabatic optical power transfer according to the present invention.

FIGS. 10A and 10B are top views of a waveguide and fiber-optic-tapersegment adapted and assembled for substantially adiabatic optical powertransfer according to the present invention.

FIGS. 11A and 11B are top views of a waveguide and fiber-optic-tapersegment adapted and assembled for substantially adiabatic optical powertransfer according to the present invention.

FIG. 12 is a top view of a waveguide and fiber-optic-taper segmentadapted and assembled for substantially adiabatic optical power transferaccording to the present invention.

FIGS. 13A and 13B are top and side views, respectively, of waveguidesadapted for substantially adiabatic optical power transfer according tothe present invention.

FIGS. 14A and 14B are side and bottom views, respectively, of an opticaldevice including waveguides adapted for substantially adiabatic opticalpower transfer according to the present invention.

FIGS. 15A and 15B are top and side views, respectively, of an assembledoptical device including waveguides adapted for substantially adiabaticoptical power transfer according to the present invention.

FIGS. 16A and 16B are top and side views, respectively, of a waveguideadapted for substantially adiabatic optical power transfer according tothe present invention.

FIGS. 17A and 17B are side and bottom views, respectively, of afiber-optic adapted for substantially adiabatic optical power transferaccording to the present invention.

FIGS. 18A and 18B are top and side views, respectively, of an assembledoptical device including a waveguide and a fiber-optic adapted forsubstantially adiabatic optical power transfer according to the presentinvention.

FIGS. 19A and 19B are top and side schematic diagrams, respectively, ofan optical device including an integrated external-transfer waveguideaccording to the present invention. FIGS. 19C and 19D are top and sideschematic diagrams, respectively, of an optical device including anintegrated external-transfer optical waveguide according to the presentinvention.

FIGS. 20A and 20B are top and side views, respectively, of an exemplaryoptical device including an integrated external-transfer opticalwaveguide according to the present invention. FIGS. 20C and 20D are topand side views, respectively, of an exemplary optical device includingan integrated external-transfer optical waveguide according to thepresent invention. FIGS. 20E and 20F are top and side views,respectively, of an exemplary optical device including an integratedexternal-transfer optical waveguide according to the present invention.FIGS. 20G and 20H are top and side views, respectively, of an exemplaryoptical device including an integrated external-transfer opticalwaveguide according to the present invention.

FIG. 21 is a side view of an exemplary external-transfer opticalwaveguide according to the present invention.

FIGS. 22A and 22B are top and side views, respectively, of an exemplaryoptical device including integrated external-transfer optical waveguidesaccording to the present invention.

FIG. 23 is a top view of an exemplary external-transfer opticalwaveguide according to the present invention.

FIG. 24 is a top view of an exemplary optical device includingintegrated external-transfer optical waveguides according to the presentinvention.

FIG. 25 is a top view of an exemplary optical device including anintegrated external-transfer optical waveguide according to the presentinvention.

FIGS. 26A and 26B are isometric views of exemplary optical devicesincluding integrated external-transfer optical waveguides according tothe present invention.

FIGS. 27A and 27B are isometric views of exemplary optical devicesincluding integrated external-transfer optical waveguides according tothe present invention.

FIGS. 28A and 28B are isometric views of exemplary optical devicesincluding integrated external-transfer optical waveguides according tothe present invention.

FIGS. 29A, 29B, 29C, and 29D are isometric views of exemplary opticaldevices including integrated external-transfer optical waveguidesaccording to the present invention.

FIG. 30 is an isometric view of an exemplary optical device includingintegrated external-transfer optical waveguides according to the presentinvention.

FIG. 31 is an isometric view of an exemplary optical device includingintegrated external-transfer optical waveguides according to the presentinvention.

FIG. 32 is an isometric view of an exemplary optical device includingintegrated external-transfer optical waveguides according to the presentinvention.

FIGS. 33A and 33B are isometric views of exemplary optical devicesincluding integrated external-transfer optical waveguides according tothe present invention.

FIGS. 34A and 34B are side views of an optical device includingintegrated external-transfer optical waveguides according to the presentinvention.

FIGS. 35A and 35B are top views of exemplary optical devices includingintegrated external-transfer optical waveguide according to the presentinvention.

FIGS. 36A and 36B are side views of an optical device includingintegrated external-transfer optical waveguides according to the presentinvention.

FIGS. 37A and 37B are side views of an optical device includingintegrated external-transfer optical waveguides according to the presentinvention.

FIG. 38 is a top view of an exemplary optical device includingintegrated external-transfer optical waveguides according to the presentinvention.

FIG. 39 is a side view of an exemplary optical device includingintegrated external-transfer optical waveguides according to the presentinvention.

FIG. 40 is a process diagram (cross-section) illustrating fabrication ofan external-transfer optical waveguide or a planar transmission opticalwaveguide according to the present invention.

FIG. 41 is a process diagram (plan) illustrating fabrication on a planarwaveguide substrate including planar transmission optical waveguidesaccording to the present invention.

FIGS. 42A and 42B are plan views of exemplary optical devices assembledonto a planar waveguide substrate according to the present invention.

FIGS. 43A and 43B are plan views of exemplary optical devices assembledonto a planar waveguide substrate according to the present invention.

FIGS. 44A and 44B are plan views of exemplary optical devices assembledonto a planar waveguide substrate according to the present invention.

FIGS. 45A and 45B are plan and side views of exemplary optical junctionsadapted and assembled for transverse-transfer according to the presentinvention.

FIG. 46 is a plot of optical power transfer between waveguides as afunction of transverse offset for end-transfer,mode-interference-coupled transverse-transfer, and adiabatictransverse-transfer. The optical power transfer is calculated using asimple analytical model.

FIG. 47 is a plot of optical power transfer between waveguides as afunction of transverse offset, the waveguides being adapted andpositioned for adiabatic transverse-transfer according to the presentinvention. The curve (with solid triangles) is calculated using anumerical model, while the open squares are experimentally measuredvalues.

It should be noted that the relative proportions of various structuresshown in the Figures may be distorted to more clearly illustrate thepresent invention. Relative dimensions of various devices, waveguides,resonators, optical fibers/tapers, and so forth may be distorted, bothrelative to each other as well as in their relative transverse and/orlongitudinal proportions. In many of the Figures the transversedimension of an optical element is enlarged relative to the longitudinaldimension for clarity, which will cause variations of transversedimension(s) with longitudinal position to appear exaggerated. Also, inFigures which show an optical fiber positioned for end-transfer ofoptical power, the fiber diameter is typically much larger relative toother waveguide dimensions than is depicted. To show the fiberaccurately scaled could have made it larger than the drawing sheet size.On the other hand, Figures showing a fiber-optic-taper segment are muchcloser to the actual relative scale.

The embodiments shown in the Figures are exemplary, and should not beconstrued as limiting the scope of the present invention as disclosedand/or claimed herein.

DETAILED DESCRIPTION OF EMBODIMENTS

For purposes of the written description and/or claims, “index” maydenote the bulk refractive index of a particular material (also referredto herein as a “material index”) or may denote an “effective index”n_(eff), related to the propagation constant β of a particular opticalmode in a particular optical element by β=2πn_(eff)/λ. The effectiveindex may also be referred to herein as a “modal index”. As referred toherein, the term “low-index” shall denote any materials and/or opticalstructures having an index less than about 2.5, while “high-index” shalldenote any materials and/or structures having an index greater thanabout 2.5. Within these bounds, “low-index” may preferably refer to:silica (SiO_(x)), germano-silicate, boro-silicate, other doped silicas,and/or other silica-based materials; silicon nitride (Si_(x)N_(y))and/or silicon oxynitrides (SiO_(x)N_(y)); other glasses; other oxides;various polymers; and/or any other suitable optical materials havingindices between below about 2.5. “Low-index” may also include opticalfiber, optical waveguides, planar optical waveguides, and/or any otheroptical components incorporating such materials and/or exhibiting amodal index below about 2.5. Similarly, “high-index” may preferablyrefer to materials such as semiconductors, IR materials, and/or anyother suitable optical materials having indices greater than about 2.5,and/or optical waveguides of any suitable type incorporating suchmaterial and/or exhibiting a modal index greater than about 2.5. Theterms “high-index” and “low-index” are to be distinguished from theterms “lower-index” and “higher-index”, also employed herein.“Low-index” and “high-index” refer to an absolute numerical value of theindex (greater than or less than about 2.5), while “lower-index” and“higher-index” are relative terms indicating which of two particularmaterials has the larger index, regardless of the absolute numericalvalues of the indices.

The term “planar optical waveguide” as employed herein shall denote anyoptical waveguide that is provided on a substantially planar substrate.Examples of such waveguides include ridge waveguides, buried waveguides,semiconductor waveguides, other high-index waveguides, silica-basedwaveguides, polymer waveguides, other low-index waveguides, core/cladtype waveguides, multi-layer reflector waveguides, metal-cladwaveguides, air-guided waveguides, photonic crystal/photonicbandgap-based waveguides, and myriad other examples not explicitly setforth herein but nevertheless falling within the scope of inventiveconcepts disclosed and/or claimed herein. Many suitable substrates maybe employed, including semiconductor, crystalline, silica orsilica-based, other glasses, ceramic, metal, and myriad other examplesnot explicitly set forth herein but nevertheless falling within thescope of inventive concepts disclosed and/or claimed herein.

To provide optical junction apparatus and methods according to thepresent invention, transverse-transfer of optical power (adiabatic ormode-interference-coupled) between initially separate and subsequentlyassembled optical components may be exploited. Such optical junctionsmay be employed for transferring optical signal power between varioustypes of optical components used to construct assembled optical devices,systems, and/or sub-systems. Examples of initially separate componentsthat may be assembled to achieve optical power transfer therebetween mayinclude but are not limited to: two separate optical fibers; twoseparate planar waveguides; an optical fiber and a planar waveguide; anoptical device integrated onto a substrate and an optical fiber, planarwaveguide, or other optical waveguide separate from the substrate; twooptical devices integrated onto separate substrates. Of particular noteis the use of apparatus and methods according to the present inventionfor transferring optical power between an optical transmission system(particularly a fiber-optic transmission system) and asemiconductor-based optical device on a substrate. Other combinations ofseparate optical components may be optically coupled using apparatus andmethods adapted for transverse-transfer of optical power (adiabatic ormode-interference-coupled) according to the present invention. Some sortof joining element may typically be employed to effect assembly ofinitially separate and subsequently assembled components. Examples ofsuch a joining element (which might arise from an interaction betweenthe components and/or from structural members associated with one orboth components) may include, but are not limited to: retainer, clamp,fastener, an adhesive, solder, potting or embedding materials, clip,tab/slot, groove, optical contacting, electrostatic and/or magnetostaticforces (including MEMS-based devices), spring or micro-spring, hermeticor micro-hermetic sealing of the assembled components, wafer-bondingtechniques. Optical devices may be constructed in which various aspectsof the functionality of the optical device reside on initially separateoptical components, with the optical device becoming fully functionalupon assembling and establishing an optical junction between theseparate components. Device functionality may be provided, altered,and/or controlled via optical junctions according to the presentinvention.

Multiple planar optical waveguides may be provided on a common substrateto form so-called planar waveguide circuits, sometimes also referred toas planar lightwave circuits (PLCs), optical integrated circuits, oropto-electronic integrated circuits. The multiple planar waveguides mayall be provided at the same height or thickness above the underlyingwaveguide substrate, or may be provided at multiple heights orthicknesses above the waveguide substrate to form a three-dimensionaloptical network. Providing multiple planar waveguides together on asingle waveguide substrate enables construction of composite opticalassemblies including multiple optical devices connected in any suitabletopology. Planar waveguides and planar waveguide circuits comprise animportant class of transmission optical waveguides used to implement thepresent invention. A planar waveguide may often comprise a low-indexcore/cladding-type dielectric waveguide fabricated on a substantiallyplanar substrate, often silica or silica-based waveguides fabricated onan oxide-coated silicon substrate. Silicon is a desirable waveguidesubstrate material for a variety of reasons, including but not limitedto: relatively easy and well-understood material processing techniques;mature industry standards; highly planar single crystal facesobtainable; amenable to selective dry- and/or wet-etching; highly rigid;desirable thermal characteristics. The silicon substrate is oftenprovided with a silica over-layer, with one or more planar waveguides onthe silica over-layer. Silica and silica-based materials are nearlyideal and well-understood optical materials.

Substantially adiabatic transverse-transfer of optical power may beemployed to provide reduced alignment-sensitivity when establishing anoptical junction. In order to achieve substantially adiabatic opticalpower transfer, at least one of the joined optical components (typicallywaveguides) must have a modal index varying along the length of thejunction region, thereby adapting the waveguide for substantiallyadiabatic transverse-transfer. This modal index variation may beachieved in a variety of ways: 1) one or both transverse dimensions ofthe waveguide core and/or cladding may vary along the length of thewaveguide; 2) the index of core and/or cladding material may vary alongthe length of the waveguide; 3) material of a selected index may beplaced near the waveguide in amounts and/or at distances varying alongthe length of the waveguide; 4) a chirped grating may be written alongthe waveguide and optical material of differing index may be deposited,yielding an effective index varying along the length of the waveguide.Other techniques may be devised, and any suitable technique may be usedalone or in combination to produce waveguides adapted for substantiallyadiabatic optical power transfer for providing optical junctionsaccording to the present invention.

For purposes of the present written description and/or claims, theadiabatic condition shall generally be defined as longitudinal variationof one or more waveguide properties sufficiently gradual so as tomaintain near or below an operationally acceptable level optical powertransfer between an isolated or system mode of interest and anotherisolated, system, or radiation mode. An operationally acceptable levelmay be defined by any relevant set or subset of applicable constraintsand/or requirements arising from the performance, fabrication, deviceyield, assembly, testing, availability, cost, supply, demand, and/orother factors surrounding the manufacture and deployment of a particularassembled optical device.

FIGS. 1A, 1B, 1C, 2A, 2B, 2C, 3A, 3B, and 3C schematically illustrate apair of stacked waveguides 110 and 120 (i.e., surface-joined), withwaveguide 110 shown positioned on substrate 111 and waveguide 120 onsubstrate 121 (substrate 121 omitted from FIGS. 1A, 2A, and 3A forclarity). Each waveguide core 112/122 varies in width along the lengthof the respective waveguide over junction region 101. The waveguidecores may equivalently vary in height, or in both transverse dimensions,along the length of a junction region (not shown here). The variation incore dimension(s) results in corresponding variation in Δβ along thelengths of the waveguides, which are preferably configured so that Δβ=0at some point in the junction region 101. Optical signal power enteringwaveguide 110 (from elsewhere on substrate 111) is transferredsubstantially completely to waveguide 120, provided that: 1) |κ|<<|Δβ|at the ends of junction region 101; 2) κ is sufficiently large in thecentral portions of junction region 101; and 3) Δβ is positive near oneend of the junction region and negative near the other end.Alternatively, optical signal power entering through waveguide 120 maybe substantially completely transferred to waveguide 110, (and hence toother components optically integrated with waveguide 110, if present).Exemplary dimensions for dielectric waveguides 110 and 120 might be:maximum core width 1-10 μm, preferably between about 2-5 μm; length ofjunction region 101 between about 100 and 2000 μm, typically severalhundred μm; spacing between waveguide cores between about 0-3 μm.

The behavior of adiabatic optical power transfer with respect tovariation in κ means that substantially complete transfer of opticalpower between waveguide 110 and 120 may be achieved within a wider rangeof relative position of waveguides 110 and 120 than could be toleratedwith end-coupled or mode-interference-coupled devices. Waveguide 120 maytherefore be positioned using passive alignment techniques, such as:providing alignment structures (not shown) on substrate 111 forreceiving and positioning waveguide 120 relative to waveguide 110 (as inearlier-cited application A9, for example); providing waveguide 120 on asubstrate 121 adapted for mating with substrate 111, so as properlyposition waveguides 110 and 120; or by other means. Active alignmenttechniques may also be employed (such as machine-vision assemblytechniques, or by monitoring optical behavior of the coupledwaveguides), and would be more readily and economically implemented withrelaxed alignment tolerances as provided by the present invention.

In FIGS. 1A, 1B, and 1C, approach regions 102/103 are each shown withwaveguide 120 or 110, respectively, bilaterally tapering in a horizontaldimension before finally coming to an end. An isolated mode of waveguide110, for example, would be minimally perturbed by the appearance of thenarrow tip at the end of waveguide 120. The increase in width ofwaveguide 120 along approach region 102 is preferably sufficientlygradual so as to substantially avoid undesirable optical power transferto other system modes (i.e., adiabatic increase in width). Similarly,waveguide 110 bilaterally tapers along approach region 103 before endingin a narrow tip, where it minimally perturbs an isolated mode ofwaveguide 120. FIGS. 2A, 2B, and 2C show a similar arrangement ofapproach regions 102/103 where waveguides are horizontally tapered fromonly one side (equivalently, beveled). FIGS. 3A, 3B, and 3C show anarrangement where waveguides 110/120 are vertically tapered or beveledalong the approach regions 103/102. In this embodiment the adiabaticcondition is achieved primarily by the gradual approach of the taperedwaveguide surface closer and closer to the other waveguide. Thesearrangements are exemplary, and many other arrangements may beequivalently employed for maintaining the adiabatic condition along theapproach regions. It should also be noted that the division between“approach regions” and the “junction region” in these and subsequentexamples need not be sharply delineated, either structurally orfunctionally, and in fact to satisfy the adiabatic condition theapproach regions and junction region should preferably smoothlytransition from one to the other.

FIGS. 4A, 4B, 4C, 5A, 5B, and 5C schematically illustrate a pair ofwaveguides 410 and 420 side by side (i.e., side-joined), with waveguide410 shown positioned on substrate 411 and waveguide 420 on substrate 421(substrate 421 omitted from FIGS. 4A and 5A for clarity). Each waveguidecore 412/422 varies in width along the length of the respectivewaveguide over junction region 401 (or may equivalently vary in height,or in both transverse dimensions, along the length of a junction region;not shown here). As in the previous examples, the variation in coredimension(s) results in corresponding variation in Δβ along the lengthsof the waveguides, which are preferably configured so that Δβ=0 at somepoint in the junction region 401. Substantially complete transfer ofoptical signal power may be achieved provided that: 1) |κ|<<|Δβ⊕ at theends of junction region 401; 2) κ is sufficiently large in the centralportions of junction region 401; and 3) Δβ is positive near one end ofthe junction region and negative near the other end. Position tolerancesfor achieving substantially complete optical power transfer are relaxedin these embodiments relative to mode-interference-coupled waveguides ina manner similar to that described for earlier examples, and theseside-joined examples may be aligned/assembled employing similartechniques.

In FIGS. 4A, 4B, and 4C, approach regions 402/403 are each shown withwaveguide 420 or 410, respectively, tapered or beveled the horizontaldimension before finally coming to an end at a narrow tip substantiallyin contact with the other waveguide. An isolated mode of waveguide 410,for example, would be minimally perturbed by the appearance of thenarrow tip at the end of waveguide 420. The increase in width ofwaveguide 420 along approach region 402 is preferably sufficientlygradual so as to substantially avoid undesirable optical power transferto other system modes (i.e., adiabatic increase in width). Similarly,waveguide 410 tapers along approach region 403 before ending in a narrowtip substantially in contact with waveguide 420, where it minimallyperturbs an isolated mode of waveguide 420. FIGS. 5A, 5B, and 5C show anarrangement where waveguides 410/420 are horizontally tapered or beveledalong the approach regions 403/402, but with the narrow tip of eachwaveguide positioned away from the other waveguide. In this embodimentthe adiabatic condition is achieved primarily by the gradual approach ofthe tapered waveguide surface closer and closer to the other waveguide.These arrangements are exemplary, and many other arrangements may beequivalently employed for maintaining the adiabatic condition along theapproach regions.

FIGS. 6A and 6B show waveguide 610 on substrate 611 side-joined (FIG.6A) or surface-joined (FIG. 6B) to waveguide 620 on substrate 621(omitted from FIG. 6A for clarity). In each of these examples variationof the modal indices of the waveguides is achieved by a longitudinalgradient in the index differential between waveguide cores 612/622 andthe respective waveguide cladding. In each waveguide, the indexdifferential may be a maximum at one end (input/output ends ofwaveguides 610/620) and decreases to substantially zero across thejunction region. The index differential gradient may arise from acladding index gradient, a core index gradient (as in FIGS. 6A and 6B),or both. Any of the specific examples of coupled waveguides disclosedherein, or equivalents thereof, may be implemented with suchgraded-index-differential waveguides.

The longitudinal modal index variations shown in the embodiments ofFIGS. 1A through 6B may be readily achieved using standardspatially-selective material processing techniques, including but notlimited to lithography, deposition, doping, implantation, masking,etching, optical densification, photochemistry, and so forth. Thesetechniques may also be employed to fabricate other optical components ona substrate along with one of the waveguides adapted for adiabaticoptical power transfer, thereby enabling transfer of optical power fromvarious components integrated onto a substrate to a separate waveguide.Use of such an external-transfer waveguide was discussed extensively inearlier-cited application A8 (wherein the “external-transfer waveguide”was referred to as an “external-coupling waveguide”; these phrases maybe considered equivalent for purposes of the present written descriptionand/or claims), and is discussed in detail hereinbelow. Application A8discloses use of an external-transfer waveguide end-coupled to anoptical component on a common substrate and mode-interference coupled toa separate transmission waveguide. All of the methods, apparatus, andembodiments disclosed in earlier-cited application A8 may be modified toinclude an external-transfer waveguide that is instead adapted forsubstantially adiabatic optical power transfer to the separatetransmission optical waveguide. The fabrication, assembly, and alignmenttolerances are thereby relaxed, and construction of such devices may becorrespondingly less difficult, expensive, and/or time-consuming.

The degree to which substantially adiabatic transverse-transfer ofoptical power depends on waveguide position may be most readilycharacterized in relation to transverse optical mode sizescharacteristic of the respective waveguides. In many cases oftransverse-transfer, sufficiently accurate and precise relativepositioning of the waveguides may be readily achieved in one of thetransverse dimensions (vertical or horizontal/lateral relative to thesubstrate), even if the position tolerance is only a fraction of thecorresponding transverse optical mode sizes characteristic of thewaveguides (for example, position tolerance less than about 0.5 timestransverse mode size in the corresponding transverse dimension). In theexemplary embodiments of FIGS. 1A-3C, vertical relative positioning maybe determined by mechanical contact between the surface-joinedwaveguides, between one waveguide and support/alignment structuresprovided on the other waveguide substrate, and/or betweensupport/alignment structures on each waveguide substrate. The accuracyof such positioning is dependent on the precision material fabricationand processing techniques employed to fabricate the waveguides, and maytherefore provide sufficient accuracy and precision for achievingreliable and reproducible transverse-transfer of optical power, evenwithin tolerances that are only a fraction of the vertical transversemode size. Similarly, in the exemplary embodiments of FIGS. 4A-6B,precision fabrication of the waveguides enables precision lateral orhorizontal positioning by mechanical contact between side-joinedwaveguides, even to within a fraction of the horizontal transverse modesize. It is often the case that similarly accurate relative positioningof the waveguides in the other transverse dimension (horizontal, orlateral positioning in FIGS. 1A-3C; vertical positioning for FIGS.4A-6B) may not be so readily achieved.

However, it has been demonstrated for substantially adiabatictransverse-transfer that substantially complete transfer of opticalpower between waveguides may be achieved within lateral positiontolerances that are as large as, or even twice as large as, the lateraltransverse optical mode sizes characteristic of the waveguides. As shownby a simple analytic model which generates the curves in FIG. 46,variation in the degree of optical power transfer with respect torelative transverse offset of the waveguides (in the lateral dimensionfor surface-joined waveguides) is substantially flat over a significantrange for adiabatic transverse-transfer. The corresponding degrees ofoptical power transfer for mode-interference-coupled transverse-transferand end-transfer vary more rapidly with transverse position. Thesecalculations (based on a simple analytic model) are based on waveguidecores about 5 μm wide by about 3 μm thick, vertically spaced by about 1μm of cladding, having a core index of about 1.5 and a cladding index ofabout 1.45. The core widths vary by about 20% over a junction regionabout 750 μm long for the adiabatic case. Lateral offset of thewaveguides away from the maximum power transfer position by about ±0.3times the lateral transverse mode size results in a transverse-offsetoptical transfer loss of about 0.5 dB relative to the maximum powertransfer achievable for end-coupled power transfer (typically limited byspatial mode mismatch). The transverse mode size is defined herein asthe 1/e² half-width for the intensity, with a root-mean-square mode sizebeing the relevant mode size when dissimilarly sized optical modes aresupported by the waveguides. The optical junction length for themode-interference-coupled case is chosen to yield substantially completeoptical power transfer assuming the waveguides are properlyindex-matched. For mode-interference-coupled transverse-transfer, alateral waveguide offset of about ±0.7 times the lateral transverse modesize results in an analogous 0.5 dB transverse-offset transfer loss(relative to the maximum achievable transverse-transfer; in principle,substantially complete optical power transfer). The transverse-offsettransfer loss stays below about 0.5 dB, and is substantially flat, forlateral offsets as large as about 1.5 times the transverse mode size foradiabatic transverse-transfer. The flatness of the transverse-offsettransfer loss can be characterized as remaining within a specific rangearound a nominal optical power transfer loss over an acceptable range oftransverse offset (within 0.5 dB of 100% transfer in this example).

These values depend on the specific geometry of the waveguides employedfor transverse-transfer of optical power, the core and claddingmaterials employed, and the size of the coupling constant κ. The offsettolerance for mode-interference-coupled transverse-transfer also dependson the accuracy with which the required interaction length may beachieved. For adiabatic transverse-transfer, the offset tolerance alsodepends on the length of the optical junction region and the variationof κ and Δβ along the waveguides in the junction region. For comparablevalues of the coupling constant κ, the results of FIG. 46 are achievedwith an optical junction region about 5 times longer for adiabatictransverse-transfer than that required for mode-interference-coupledtransverse-transfer. Larger lateral offsets can be tolerated with evenlonger junction regions for adiabatic transverse-transfer and/or withincreasing values of κ, assuming that care is taken to maintain asubstantially adiabatic condition along the waveguides.

Employing waveguide cores having large aspect ratios may enhance thetransverse-offset-insensitivity exhibited by adiabatictransverse-transfer. A numerically modeled curve and measured data areshown in FIG. 47 for optical power transfer between surface-joinedwaveguides with thin-film type cores. The waveguides each have a coredecreasing from about 2.2 μm wide to about 0.5 μm wide over about a 250μm long optical junction region. The cores are about 0.13 μm thickvertically separated by about 0.8 μm of cladding, with a core index ofabout 2.0 and a cladding index of about 1.45. The cores are about 1.4 μmwide and the resulting optical mode sizes are about 0.7 μm in thelateral dimension near the point where Δβ crosses zero in the opticaljunction region. Transverse-offset optical power transfer loss remainsbelow about 0.5 dB (and is substantially flat) over a range of lateraloffsets of ±1.5 μm, or about ±2 times the corresponding transverseoptical mode size.

For various optical junctions described herein employing substantiallymode-interference-coupled transverse-transfer of optical power, thetransverse-offset optical power transfer losses may preferably remainbelow about 0.5 dB for transverse offsets at least as large as about±0.5 times the corresponding transverse optical mode size characteristicof one of the waveguides. For various optical junctions described hereinemploying substantially adiabatic transverse-transfer of optical power,the transverse-offset optical power transfer losses may preferablyremain below about 0.5 dB for transverse offsets at least as large asabout ±1 times the corresponding transverse optical mode size, and mostpreferably at least as large as about ±1.5 times the correspondingtransverse optical mode size. For various optical junctions describedherein employing substantially adiabatic transverse-transfer of opticalpower, transverse-offset transfer loss characterized as “flat” shallpreferably remain within ±0.5 dB of a nominal transfer loss, mostpreferably within ±0.3 dB, over transverse offsets at least as large asabout ±1 times the corresponding transverse optical mode size, and mostpreferably at least as large as about ±1.5 times the correspondingtransverse optical mode size. Experimental and numerical data shown inFIG. 47 show that these criteria may be readily achieved.

Waveguides for implementing alignment-insensitive optical junctionsaccording to the present invention may be provided by mechanicalmodification of more standard waveguide structures (i.e., waveguide,optical fiber, or similar structure having core and/or cladding profilesinitially having substantially no longitudinal variation). By etching orpolishing a standard waveguide at a very shallow angle, a beveledcoupling region may be created having a longitudinally varying modalindex. Placement of this etched or polished surface against anotherwaveguide may then enable substantially adiabatic optical power transferbetween the waveguides, if the termination conditions are met. Side- andsurface-joined examples are shown in FIGS. 7A and 7B (side-joined) and8A and 8B (surface-joined). A waveguide 710 on substrate 711 andincluding a core 712 is etched at a shallow angle (for example, about0.5°, resulting in a junction region about 600 μm long for a waveguide 5μm wide; other angles and junction region lengths may be employed). Asecond waveguide 720 with core 722 on substrate 721 (omitted from FIG.7A for clarity) is similarly etched or polished and positioned relativeto waveguide 710 as shown, preferably with the etched/polished surfacesof the waveguides in contact. As with embodiments previously shown, thenature of adiabatic optical power transfer results in substantiallycomplete transfer of optical signal power between the waveguides for awide range of relative position of waveguides 710 and 720, bothhorizontal and vertical. Passive and/or low-precision assembly/alignmenttechniques may therefore be readily and/or economically implemented forconstructing devices, as described hereinabove. Sufficiently gradualapproach conditions may be readily achieved, since cladding layers maybe arbitrarily thick, with thicker cladding reducing perturbation of anisolated mode of one waveguide by the appearance of the narrow tip ofthe other waveguide.

One way to help ensure substantially complete transfer of optical powerbetween waveguides is to employ two substantially identical waveguides(with opposing modal index gradients, of course), as has been the casein the embodiments presented thus far. This ensures that there will be apoint within the junction region with Δβ=0 and that Δβ will haveopposite signs at the ends of the junction region (necessary conditionsfor substantially alignment-insensitive, substantially completeadiabatic optical power transfer), thereby simplifying the designprocess. Such a symmetric arrangement also ensures that if the inputtermination condition is satisfied, then the output termination will besimilarly satisfied. However, without departing from inventive conceptsdisclosed and/or claimed herein, alignment-insensitive optical junctionapparatus and methods employing adiabatic optical power transfer may beimplemented using differing waveguides. Such waveguides may be of thesame general type but differ in size, material, index, longitudinalgradient, and so on. Alternatively, the waveguides may be ofsubstantially dissimilar types, as long as the appropriate terminationand adiabatic conditions are substantially met. It should beparticularly noted that both waveguides need not have a longitudinalmodal-index gradient. Adiabatic transverse-transfer may be implementedin methods and apparatus according to the present invention whereineither one or both waveguides have a longitudinal modal-index gradient.

In FIGS. 9A, 9B, and 9C, an optical fiber 920 with core 922 is shownpolished at a shallow angle to yield a beveled region having alongitudinally varying modal index. For example, a single-mode opticalfiber with about an 8 μm core diameter and about a 0.5° polishing anglemay yield a junction region about 1 mm in length; other angles andcoupling region lengths may be employed. Optical fiber 920 is shownsurface-joined to waveguide 910 with core 912 on substrate 911.Side-joining could also be employed, but may prove mechanicallyinconvenient due to the diameter of the cladding of optical fiber 920.Waveguide 910 is shown having no longitudinal variation of modal index.The modal index of waveguide 910 should be larger than the index of thecladding layer of fiber 920, but smaller than the modal index of thefull fiber 920 (complete core 912 plus complete cladding).Alternatively, waveguide 910 may be provided with alongitudinally-varying modal index in any suitable manner, includingthose illustrated in FIGS. 1A through 8B.

FIGS. 10A, 10B, 11A, 11B, and 12 show a fiber-optic-taper segment andvarious waveguides adapted for substantially adiabatic optical powertransfer. In FIGS. 10A and 10B, taper segment 1020 is adiabaticallysurface-joined (FIG. 10A) or side-joined (FIG. 10B) to waveguide 1010,the waveguide 1010 including a longitudinally tapered core 1012. Themodal index of waveguide 1010 is preferably larger than the modal indexof taper segment 1020, while the index of the cladding of waveguide 1010is preferably smaller than the modal index of the taper segment 1020. InFIGS. 11A and 11B, taper segment 1120 is adiabatically surface-joined(FIG. 11A) or side-joined (FIG. 11B) to waveguide 1110, the waveguide1110 including core 1112 with a longitudinal index-differentialgradient. The modal index of waveguide 1110 is preferably larger thanthe modal index of taper segment 1120, while the index of the claddingof waveguide 1110 is preferably smaller than the modal index of thetaper segment 1120. FIG. 12 shows fiber-optic-taper segment 1220adiabatically side-joined to side-etched (or side-polished) beveledwaveguide 1210. Surface-joining could also be employed with an etchedwaveguide and taper segment, but may prove to be mechanicallyinconvenient. The modal index of waveguide 1210 is preferably largerthan the modal index of taper segment 1220, while the index of thecladding of waveguide 1210 is preferably smaller than the modal index ofthe taper segment 1220. It should be noted that the portion of thefiber-optic-taper segment 1020/1120/1220 shown forming an adiabaticoptical power transfer junction in these embodiments is preferably thesubstantially straight central portion of the fiber-optic taper, andtypically has a diameter and modal index substantially invariant withrespect to longitudinal position along the taper segment. It may bedesirable to construct adiabatic optical power transfer devices usingportions of a fiber-optic taper having longitudinally varying diameterand/or modal index.

In any of the embodiments of FIGS. 10A through 12, the waveguide1010/1110/1210 may be positioned on a substrate 1011/1111/1211, andperhaps also integrated with other optical components thereon. Alignmentstructures of the sort described in earlier-cited application A9 may beemployed for aligning the fiber-optic-taper segment 1020/1120/1220relative to the waveguide. The nature of adiabatic optical powertransfer ensures that transfer of optical power between thefiber-optic-taper segment and the waveguide is substantially insensitiveto the precise alignment of the fiber-optic-taper segment within a rangeof several microns. Construction of devices employing substantiallyadiabatic optical power transfer between a fiber-optic taper and awaveguide may be correspondingly less difficult, expensive, and/ortime-consuming than analogous devices incorporating fiber-optic tapermode-interference-coupled to the waveguide. Mode-interference-coupleddevices may nevertheless be employed, and may even be preferred undercertain circumstances, as discussed hereinbelow.

FIGS. 13A, 13B, 14A, 14B, 15A, and 15B illustrate assembly of opticalwaveguides for adiabatic optical power transfer, using so-called“flip-chip” structures and planar waveguides. FIGS. 13A and 13B show topand side views, respectively, of a portion of a planar waveguide circuiton a substrate 1311, including planar waveguides 1310 a and 1310 b withtapered waveguide cores 1312 a and 1312 b, respectively. Tapered cores1312 a and 1312 b are shown tapering in both transverse dimensions forclarity, but may equivalently be provided tapering in only onetransverse dimension, with a longitudinal index-differential gradient,or in any other suitable form for providing a longitudinally varyingmodal index along a junction region. Substrate 1311 is provided withalignment guides 1315, and electrical contact 1316. The planar waveguidecircuit is adapted to receive an optical component and transmit opticalpower thereto/therefrom through waveguides 1310 a/1310 b. FIGS. 14A and14B show side and bottom views, respectively, of an optical device 1324on a substrate 1321. Device 1324 may be an active or passive device ofany desired sort, including a laser, modulator, filter, or other opticaldevice. External-transfer waveguides 1320 a and 1320 b are alsopositioned on substrate 1321 and are end-coupled to device 1324, in themanner disclosed in earlier-cited application A8 (wherein theexternal-transfer waveguides are referred to as external-couplingwaveguides; the terms may be considered equivalent for purposes of thepresent written description and/or claims). The external-transferwaveguides 1320 a/1320 b are provided with cores 1322 a/1322 b(respectively), each having a longitudinally varying modal indexprovided in any suitable manner as described hereinabove. Optical device1324 may be provided with an electrical contact 1326 for engagingcontact 1316.

FIGS. 15A and 15B show top and side views, respectively, of the device1324 on substrate 1321 assembled with waveguide substrate 1311.Alignment guides 1315 engage lithographically defined alignment edges ofdevice substrate 1321 so as to position external-transfer waveguides1320 a/1320 b for adiabatic optical power transfer with planarwaveguides 1310 a/1310 b (respectively). Many other forms of alignmentguides may be equivalently employed, including but not limited toridges, edges, tabs, slots, pins, flanges, interlocking teeth, opticaltargets, and so forth. An optical signal may therefore be substantiallycompletely transferred from a planar waveguide into the device, andsubstantially completely transferred into the other planar waveguideafter being acted upon by the device. Alternatively, optical signalpower emitted from device 1324 may be transferred into external-transferwaveguides 1310 a/1310 b. The relaxed tolerances of adiabatictransverse-transfer enable an optical device to be readily inserted intoa planar waveguide system with substantially reduced time, cost, and/ordifficulty, and/or increased device yield. In particular, relaxedtolerances readily enable simultaneous alignment of an optical devicewith multiple waveguides, as exemplified in FIGS. 15A and 15B. Suchsimultaneous alignment would be exceedingly difficult to achieve byeither end-coupling or mode-interference coupling. Multiple devicelocations such as that shown in FIGS. 13A and 13B may be provided on asingle waveguide substrate, thereby enabling ready assembly of multipleoptical devices provided in the manner shown in FIGS. 14A and 14B.

It should be re-emphasized that any of the methods and apparatusdisclosed in earlier-cited application A8 may be implemented usingwaveguides adapted for substantially adiabatic optical power transfer asdisclosed herein as an external-transfer waveguide (external-transferwaveguides are referred to as external-coupling waveguides inearlier-cited application A8; the terms may be considered equivalent forpurposes of the present written description and/or claims). The relaxedalignment tolerances provided by adiabatic optical power transfer wouldfurther enhance the advantages provided by the use of external-transferwaveguides according to application A8.

FIGS. 16A, 16B, 17A, 17B, 18A, and 18B illustrate an assembly suitablefor substantially adiabatic optical signal power transfer between aplanar waveguide system and an optical fiber. FIGS. 16A and 16B are topand side views, respectively, of a portion of a planar waveguidesubstrate 1611 adapted for substantially adiabatic optical powertransfer to an optical fiber. Waveguide 1610 may be provided onsubstrate 1611 with tapering core 1612. The longitudinally varying modalindex provided by tapered core 1612 may be provided by any othersuitable structure, including a core tapered in one or both transversedimensions, a graded-index-differential core, or any other suitableform. Substrate 1611 is provided with alignment guides 1615. FIGS. 17Aand 17B are side and bottom views, respectively, of an angle-polishedbeveled optical fiber 1620 positioned in an angled V-block 1621. Theangled V-groove 1623 of block 1621 may preferably be used to polishoptical fiber 1620 to the appropriate bevel angle, and may then be usedto position the beveled end of fiber 1620 and core 1622 relative towaveguide 1610.

FIGS. 18A and 18B are top and side views, respectively, of planarwaveguide substrate 1611 assembled with block 1621. Alignment guides1615 engage lithographically defined alignment edges of block 1621 so asto position the polished portion of fiber 1620 relative to tapered core1612 for substantially adiabatic optical power transfer therebetween.Many other forms of alignment guides may be equivalently employed,including but not limited to ridges, edges, tabs, slots, pins, flanges,interlocking teeth, optical targets, and so forth. The relaxedtolerances of adiabatic optical power transfer enable an optical fiberto be readily coupled to a planar waveguide system with substantiallyreduced time, cost, and/or difficulty, and/or increased device yield.Apparatus and methods according to the present invention may beparticularly well-suited for simultaneous assembly/alignment of anoptical component or sub-assembly with two or more other opticalwaveguides, devices, and/or sub-assemblies.

A variety of waveguide types may be employed as a waveguide adapted forsubstantially adiabatic optical power transfer while remaining withinthe scope of the present invention. A low-index dielectric waveguideincluding a core and lower-index cladding layers may be a preferredwaveguide adapted for substantially adiabatic optical power transfer(and have been shown in the Figures). Such waveguides may be fabricatedusing silica-based materials using precision material processingtechniques. The resulting waveguide may be readily employed foradiabatic optical power transfer to a silica-based fiber-optic-tapersegment or angle-polished optical fiber. Other waveguide materialsand/or configurations may be equivalently employed, and varyingwaveguide properties exploited to modify and/or enhance the utility ofadiabatic optical power transfer. Germano-silicates are commonlyemployed as waveguide or fiber core materials, and may be suitable foruse as a core material in the present application. Silicon nitride(Si_(x)N_(y)) and/or silicon oxynitride (SiO_(x)N_(y)) may also bepreferred materials for forming a waveguide core adapted for adiabatictransverse-transfer. Waveguides may be employed wherein confinement ofwaveguide optical modes is effected by: one or more waveguide coressurrounded by lower-index cladding layers, distributed Bragg reflectors,other multi-layer reflectors, photonic bandgap/photonic crystal-basedtechniques, metal reflector coatings, dielectric reflector coatings,and/or internal reflection at an air/waveguide interface. A waveguidewith a core may include a single core or multiple-cores, the latterwhich may be employed for altering the field distribution of a supportedisolated or system mode (so-called “field-flattening”, thereby furtherreducing alignment sensitivity of an assembled optical device).Waveguide materials may include (but are not limited to) one or more of:silica, germanosilicate, and/or other doped silicas or silica-basedmaterials; silicon nitride and/or silicon oxynitride; semi-conductormaterials; organic materials; inorganic materials; crystallinematerials; glassy or amorphous materials; polymeric materials;electro-optic materials; other low- or high-index dielectric materials;and so forth. Low-index dielectric waveguides (including silica-,germano-silicate-, silicon nitride-, and or silicon oxynitride-based,for example) and silica-based optical fiber (angle-polished and/ortapered) are particularly noted as suitable waveguides for adiabaticoptical power transverse-transfer according to the present invention.Such waveguides may exhibit little or no dependence of adiabatic opticalpower transfer performance on wavelength or polarization, and this maybe desirable for a variety of optical devices employing waveguidesadapted for substantially adiabatic optical power transfer according tothe present invention. Multi-layer waveguides as disclosed inearlier-cited applications A1 and A10 are also noted as potentiallydesirable candidates for use as waveguides adapted for substantiallyadiabatic optical power transfer. Such waveguides may exhibitsubstantial dependence of adiabatic optical power transverse-transferperformance on wavelength and/or polarization, and this may be desirablefor a variety of optical devices. Incorporation of electro-active and/ornon-linear-optical materials into a waveguide adapted for substantiallyadiabatic optical power transverse-transfer may enable active control ofperformance of the waveguide.

In the exemplary embodiments shown in the Figures, the waveguides (orcores thereof) adapted for adiabatic optical power transfer are shownterminating, and the embodiments shown are all intended for use insituations where substantially complete transfer of optical signal powerbetween waveguides is desired. The waveguides (or cores) need notnecessarily terminate in such embodiments, and embodiments in which thewaveguide and/or core does not terminate shall fall within the scope ofthe present invention as disclosed and/or claimed herein. Waveguidesadapted for adiabatic optical power transfer may also be employed forless than complete transfer of optical signal power between waveguideswhile remaining within the scope of the present invention as disclosedand/or claimed herein.

Fabrication of waveguides suitable for adiabatic optical power transfermay be facilitated by use of precision material processing techniques.This may be especially advantageous when such waveguides are providedintegrated onto a substrate along with other optical components and/ordevices that may be fabricated using similar techniques. These mayinclude precision and/or spatially-selective material processingtechniques such as lithography, deposition, doping, masking, etching,and so forth. Such techniques may be implemented on a wafer-scale toeffect simultaneous fabrication of many integrated groups of opticaldevices, components, and/or waveguides. In particular, waveguidesadapted for adiabatic optical power transfer employed asexternal-transfer waveguides for optical devices as disclosed inearlier-cited application A8 may preferably be fabricated on awafer-scale using these techniques.

Use of an external-transfer optical waveguide as a link between anoptical device fabricated on a substrate and a transmission opticalwaveguide is a powerful technique, and was introduced in earlier-citedapplications A8 and A11. Wafer-scale fabrication may be employed forsimultaneous fabrication of many optical devices, each having anoptically integrated external-transfer optical waveguide for providingan optical junction with a transmission waveguide (a planar waveguide,an optical fiber, or some other transmission optical waveguide).Precision material processing techniques readily enable properpositioning and alignment of each optical device and its respectiveexternal-transfer optical waveguide for optical power transfer (i.e.,optical integration, for end-transfer or transverse-transfer). Opticalpower transverse-transfer between the external-transfer opticalwaveguide and a transmission optical waveguide configured as an opticalprobe (as disclosed in earlier-cited applications A3, for example)allows testing and qualification of optical devices on the substratebefore the difficult, time-consuming, and expensive step of dividing thewafer into individual devices.

Relaxed alignment tolerances for optical power transverse-transfer (andparticularly for substantially adiabatic transverse-transfer) readilyenable proper positioning and alignment of the external-transfer opticalwaveguide and the transmission optical waveguide. Adiabatictransverse-transfer may be particularly appropriate in applicationswhere substantially complete transfer of optical power is desired and/orwhen substantially polarization- and/or wavelength-independent transferis desired, and generally provides the least demanding alignmenttolerances of the optical power transfer techniques discussed herein.Optical junction regions may be quite long, however, even exceeding amillimeter in length, which in turn increases overall device size.Mode-interference-coupled transverse transfer may be particularlyappropriate when polarization- and/or wavelength-dependent transfer isdesired, when active control of optical power transfer is desired, whenvarying degrees of optical power transfer are desired, and/or when ashort optical junction region (even shorter than 100 μm) and smallerdevice size is desired.

FIGS. 19A, 19B, 19C, and 19D schematically illustrate generic examplesof an optical device 1910 and a transmission optical waveguide 1920 withan external-transfer optical waveguide 1930 positioned and adapted fortransferring optical power therebetween according to the presentinvention. The optical device 1910 may typically define a spatialoptical mode 1911 only a few microns across. Such device-supported modesmay also differ between horizontal and vertical dimensions, sometimesbeing less than one micron high. An end face of a previous opticaldevice would generally comprise a cleaved end face of the material fromwhich the device was fabricated, through which optical signal powerwould be transmitted. For an optical device implemented according to thepresent invention, the end face 1914 may be etched rather than cleaved.Further material processing steps (lithography, deposition, masking,etching, and so forth) may be used to fabricate an external-transferoptical waveguide 1930 on the device substrate 1902. Precision materialprocessing techniques readily enable sub-micron tolerances in thefabrication and alignment of the external-transfer optical waveguide1930 on the device substrate 1902 relative to the spatial mode 1911defined by the device 1910. The external-transfer optical waveguide 1930may preferably be fabricated so as to support an optical mode 1931(substantially characteristic of the waveguide 1930 when isolated)substantially spatial-mode-matched to the optical mode 1911 defined bythe device 1910. As a result of this spatial-mode-matching, opticalpower may be efficiently transferred between the optical device 1910 andthe external-transfer optical waveguide 1930. Alternatively, the opticalpower may be transferred between the optical device and theexternal-transfer optical waveguide by any suitable method, includingend-transfer and transverse-transfer (adiabatic ormode-interference-coupled). Whatever scheme is employed for opticalpower transfer between the optical device and the external-transferoptical waveguide, fabrication/integration of the optical device and theexternal-transfer optical waveguide on a common substrate enablesprecision alignment and efficient, reproducible, and reliable opticalpower transfer.

The external-transfer optical waveguide 1930 is provided with atransverse-transfer optical junction segment 1934 to enable transfer ofoptical signal power between the optical device 1910 and a transmissionoptical waveguide 1920 at an optical junction segment 1924 thereof. Apreferred transmission optical waveguide may include an optical fiberhaving a tapered segment for transverse-transfer, a planar waveguide orplanar waveguide circuit fabricated on a substrate and having atransverse-transfer optical junction segment, an optical fiber oroptical waveguide having a side-etched transverse-transfer opticaljunction segment, or other suitable transmission optical waveguidehaving a suitably adapted transverse-transfer optical junction segment.The transmission optical waveguide may support an optical mode 1921(characteristic of the waveguide 1920 when isolated). The respectiveoptical junction segments 1934/1924 of the external-transfer opticalwaveguide 1930 and the transmission optical waveguide 1920 may beadapted for mode-interference-coupled optical power transverse-transfer(depicted schematically in FIGS. 19A and 19B), or adapted forsubstantially adiabatic optical power transverse-transfer (depictedschematically in FIGS. 19C and 19D, with transmission waveguide 1920shown on a substrate 1922 in FIG. 19D). Optical modes at/near thejunction regions 1924/1934 may resemble superpositions of isolatedwaveguide modes 1921/1931 or may be system modes that may notparticularly resemble isolated waveguides modes or superpositionsthereof.

The optical junction segment 1924 of the transmission waveguide 1920 maybe positioned relative to the optical junction segment 1934 of theexternal-transfer optical waveguide 1930 so as to achievetransverse-transfer of optical power between optical mode 1931 of theexternal-transfer optical waveguide 1930 and the optical mode 1921 oftransmission optical waveguide 1920. The structure, dimensions,material(s), and/or positions of the optical junction segment(s) 1924and/or 1934 may be chosen to achieve substantially modal-index-matchedtransverse transfer by providing the correct combination of couplingcoefficient κ and interaction length L for efficient transfer of opticalpower. Alternatively, the structure, dimensions, material(s), and/orpositions of the optical junction segment(s) 1924 and/or 1934 may bechosen to achieve substantially adiabatic transverse transfer byproviding the correct longitudinal variations of coupling coefficientκ(z) and modal-index mismatch Δβ(z). Alignment structures may beprovided on the optical device for enabling passive alignment of theoptical junction segment of the transmission optical waveguide relativeto the optical junction segment of the external-transfer opticalwaveguide.

The embodiments of FIG. 20A and many subsequent Figures generally depictconfigurations suitable for optical power transverse-transfer between anoptical device and a transmission optical waveguide through anexternal-transfer optical waveguide. Any of the embodiments depicted inthese Figures may be adapted for mode-interference-coupled orsubstantially adiabatic transverse-transfer between theexternal-transfer optical waveguide and the transmission opticalwaveguide, and should be considered to encompass both configurationsunless specifically stated otherwise.

Examples of an optical device connected to a transmission opticalwaveguide by an external-transfer optical waveguide are shown in FIGS.20A through 20H. A standard edge-emitting semiconductor laser 2010 isshown having a device optical mode defined by an active layer 2016 andcladding layers 2018. The laser is terminated by faces 2013 and 2014,preferably formed by etching of the device. An external-transfer opticalwaveguide 2030 is fabricated on the same substrate 2002 as thesemiconductor laser 2010. The external-transfer waveguide 2030 may beconfigured and positioned so as to support an optical mode that issubstantially spatial-mode-matched with the spatial mode of thesemiconductor laser output. Highly precise material processingtechniques (such as lithography, deposition, masking, etching, and soforth) may be employed for aligning the external-transfer opticalwaveguide 2030 to the laser output far more accurately than could beachieved by active alignment of separate components. Such techniques,which may include self-aligned processes, are often the same as orsimilar to techniques used to fabricate the semiconductor laser 2010,and may be scaled to simultaneously fabricate numerous opticallyintegrated and precision-aligned laser/waveguide devices duringwafer-scale processing.

The laser 2010 and external-transfer optical waveguide 2030 may befabricated during a single multi-step fabrication process, or duringdistinct laser- and waveguide-fabrication processes. Laser 2010 and 2030may be fabricated with similar materials (both semiconductor-based, forexample), or with dissimilar materials (semiconductor-based laser andsilica-based waveguide, for example). The demarcation between laser 2010and waveguide 2030 may be sharp or gradual. It should be noted that thedistinction between the optical device and the external-transfer opticalwaveguide may be functional and/or structural. In any of these cases,optical power emitted by the laser 2010 is efficiently end-transferredthrough face 2014 (equivalently, end-facet 2014) of the semiconductorlaser 2010 into the external-transfer optical waveguide 2030. Etchingmay be employed to form face 2014. Alternatively, any other suitablespatially-selective material processing techniques (such asspatially-selective quantum-well inter-mixing, for example) may beemployed to form face 2014 adapted for end-transfer of optical powertherethrough. It may be desirable to provide one or more optical coatinglayers on laser end facet 2013 and/or between laser end facet 2014 andthe external-transfer waveguide 2030. Such optical coatings on the endfacets of the laser may serve to modify and/or control reflectivity ofthe end facets and operational properties of the laser. Any opticalcoating layers on end facet 2014 may be applied before fabrication ofthe external-transfer waveguide 2030, or may be formed between end facet2014 and waveguide 2030 after fabrication thereof. It may be desirableto provide face 2014 as an angled end facet, and to provideexternal-transfer waveguide 2030 with an angled end face angled in acomplementary fashion, for reducing feedback into laser 2010 whileenabling efficient end-transfer between laser 2010 and external-transferwaveguide 2030.

Alternatively, an optical device and a corresponding external-transferoptical waveguide may be positioned and adapted for transverse-transferof optical power therebetween. Precision manufacturing techniques asdescribed hereinabove may be employed for achieving efficient opticalpower transverse-transfer between the optical device and theexternal-transfer optical waveguide.

Once optical power has been transferred to the external-transfer opticalwaveguide 2030, it may be transferred to a transmission opticalwaveguide 2020 (a fiber-optic taper segment in these examples) bytransverse-transfer (shown adapted for mode-interference-coupledtransverse-transfer in FIGS. 20A-20D; shown adapted for adiabatictransverse-transfer in FIGS. 20E-20H) at respective optical junctionsegments 2024/2034 of the transmission and external-transfer opticalwaveguides 2020/2030. In these exemplary devices, the external-transferoptical waveguide 2030 may preferably comprise a low-index opticalwaveguide including a core 2036 surrounded by lower-index claddinglayers 2038 (including silica, germanosilicate, silicon nitride, siliconoxynitride, other glasses, polymers, and so forth). Such a low-indexwaveguide may be passively modal-index-matched (typically through properdesign of the transverse shape/dimensions of the optical junctionsegment) to a fiber-optic-taper segment 2020 of a silica-based opticalfiber for modal-index-matched transverse transfer (FIGS. 20A-20D). Ifthe transverse shape/dimensions of the external-transfer opticalwaveguide 2030 required for spatial-mode-matching with the laser 2010and modal-index-matching with the fiber-optic-taper segment 2020 differsubstantially, the external-transfer optical waveguide 2030 may be madesufficiently long so as to enable a substantially adiabatic transitionbetween the differing shapes/dimensions. The external-transfer opticalwaveguide may alternatively be adapted for active modal-index-matchingby employing electro-active and/or non-linear optical materials andelectrical and/or optical control signals. The fiber-optic-taper segment2020 may be positioned relative to the external-transfer opticalwaveguide 2030 to achieve mode-interference-coupled transverse-transfercharacterized by a coupling coefficient κ (also determined in part bythe transverse shape/dimensions of the optical junction segment 2034 ofthe external-transfer optical waveguide 2030). The optical junctionsegments 2034 and 2024 of the external-transfer optical waveguide 2030and the fiber-optic-taper segment 2020, respectively, may preferably beconfigured to yield a coupling coefficient κ and an interaction length Lsuch that κL≈π/2, thereby resulting in substantially complete transferof optical power from the external-transfer optical waveguide to theoptical fiber.

In the exemplary embodiments of FIGS. 20E-20H, substantially adiabatictransverse-transfer may be employed between optical junction regions2024/2034, in any of the ways described hereinabove. In other exemplaryembodiments, a transmission optical waveguide of another type may beemployed instead of fiber-optic-taper segment 2020. Optical powertransverse-transfer may be established between external-transfer opticalwaveguide 2030 and a planar waveguide, for example.

The accuracy with which the optical junction segments 2024/2034 of thetransmission optical waveguide 2020 and external-transfer opticalwaveguide 2030 must be positioned to achieve a desired degree of opticalpower transfer therebetween (usually substantially complete transfer isdesired) employing mode-interference-coupled or substantially adiabatictransverse-transfer is described in detail hereinabove. Accuratetransverse positioning may be facilitated or enabled by providingalignment structures in and/or on the substrate such as grooves,flanges, posts, tabs, slots, yokes, solder/metal surface tension, andthe like for guiding the optical junction segment of the transmissionoptical waveguide (the fiber-optic-taper segment of the optical fiber inthis example) to the properly aligned position relative to the opticaljunction segment of the external-transfer optical waveguide andmaintaining the alignment thus achieved. Segments of optical fiber(separate from optical fiber carrying the optical signal) may beemployed as structural elements for aligning and supporting afiber-optic taper or other transmission waveguide, and may havefabricated thereon rings, grooves, flanges, and/or knife-edges therefor.Exemplary alignment structures 2060 are shown in FIGS. 20C/20D and200/20H for positioning fiber-optic taper segment 2020 relative toexternal-transfer waveguide 2030. Other similar alignment structures aredisclosed in earlier-cited application A9.

For mode-interference-coupled transverse-transfer, the optical junctionsegment 2034 of the external-transfer optical waveguide 2030 may beconfigured to yield the appropriate interaction length L forsubstantially complete optical power transfer (or other desired opticalpower transfer level) from the external-transfer optical waveguide 2030to the transmission optical waveguide 2020. The interaction length maybe determined by providing a bend in the external-transfer opticalwaveguide 2030 between the laser 2010 and the optical junction segment2034, with the portion of the external-transfer optical waveguide beyondthe bend being of the appropriate interaction length (FIGS. 20A-20H).Alternatively, an optical junction segment 2134 of an external-transferoptical waveguide 2130 may be provided with a raised portion of length Land the desired value of κ modal-index-matched transverse-transferto/from an optical junction segment 2124 of fiber-optic-taper segment2120 (FIG. 21). In either case, precise longitudinal positioning of thefiber-optic-taper segment relative to the external-transfer opticalwaveguide is not required. Sufficiently precise alignment of the opticalfiber (i.e., the transmission waveguide in this example) and thefiber-optic-taper segment thereof with the external-transfer opticalwaveguide (and hence with the semiconductor laser) may therefore beachieved by passive alignment techniques. The optical fiber carrying thelaser output power may be connected, spliced, or otherwise coupled to afiber-optic-based optical system by any suitable technique. Alignmentprecision and the length tolerance for the optical junction segment arefurther relaxed if adiabatic transverse-transfer is employed instead ofmode-interference-coupled transverse-transfer.

An optical device (a semiconductor laser in the preceding example)including an optically integrated external-transfer optical waveguide asdescribed hereinabove enables significant economies of manufacturing tobe realized. The use of passive alignment techniques for relativepositioning of the transmission optical waveguide and the optical devicewhile achieving high-efficiency optical power transfer therebetween is asignificant improvement over the prior art. An additional benefit is theability to pre-test and pre-qualify numerous devices/waveguidessimultaneously fabricated on a single wafer. Withcleaved-and-end-coupled devices, the wafer must be cleaved prior totesting of the devices thereon. Substantial processing time, effort, andcost are therefore expended on devices that may be subsequentlydiscarded. According to the present invention, however, each device isprovided with an external-transfer optical waveguide prior to anycleaving of the wafer, and the external-transfer optical waveguideenables optical coupling to the device for testing and characterizationprior to any division of the wafer. The devices themselves may also beused for diagnosis of neighboring devices using electrical probes andcontacts. On a wafer of laser diodes, for example, a diode may bereverse-biased to function as a photodetector for monitoring laseroutput power from a neighboring device. Alternatively, diagnosticdevices may be specifically designed into the fabrication process thatresults in the optical devices.

The description of a semiconductor laser including an opticallyintegrated external-transfer waveguide set forth in the precedingparagraphs provides only one exemplary implementation of the presentinvention. The present invention may be generalized to virtually anyother optical device that may be fabricated on a substrate. For any suchdevice, an external-transfer optical waveguide may be fabricated on thedevice substrate as an integral device component, adapted for opticalpower transfer by end-transfer and/or transverse-transfer between theexternal-transfer optical waveguide and the optical device. An opticaljunction segment of the external-transfer optical waveguide may be usedto provide efficient transfer of optical power between the device and atransmission optical waveguide. The present invention may be implementedfor single-ended or single-port devices such as the semiconductor laserof the preceding examples, and also for multi-port devices such asmodulators, filters, switches, multiplexers, splitters/combiners, and soforth. Once a device has been fabricated with the appropriate number ofinput/output segments, these may be appropriately adapted for opticalpower transfer (end- or transverse-transfer) and an external-transferoptical waveguide provided for each. Each of these external-transferoptical waveguides may then provide optical power transverse-transferbetween the optical device and respective transmission waveguides forconnection to an optical system. Optical power transverse-transferbetween each external-transfer optical waveguide and its respectivetransmission optical waveguide may be adiabatic ormode-interference-coupled (active or passive modal-index-matched), andneed not be the same for all external-transfer waveguides associatedwith the optical device.

A specific two-port example is shown in FIGS. 22A and 22B, which show asemiconductor electro-absorption modulator 2210 having input and outputexternal-transfer optical waveguides 2230 and 2231 end-coupled to inputand output faces 2214 and 2215, respectively. Modulator 2210 andexternal-transfer optical waveguides 2230/2231 are preferably fabricatedon a common substrate 2202. The external-transfer optical waveguides2230/2231 may be substantially spatial-mode-matched and aligned with anoptical mode defined by the modulator 2210. Input and outputexternal-transfer optical waveguides 2230/2231 enable optical powertransverse-transfer between the modulator 2210 and input and outputtransmission optical waveguides 2220/2221, respectively(fiber-optic-taper segments of optical fibers in this example; othertransmission optical waveguides, such as planar waveguides, couldequivalently be employed, as in FIGS. 13A through 15B). An opticalsignal to be modulated may be received from an optical system throughthe input transmission optical waveguide 2220, transferred bytransverse-transfer into the input external-transfer optical waveguide2230, transferred by end-transfer into the modulator 2210, modulated asit propagates through the modulator, transferred by end-transfer intothe output external-transfer optical waveguide 2231, transferred bytransverse-transfer into the output transmission waveguide 2221, andtransmitted to the optical system. Alternatively, optical powertransverse-transfer could be employed between modulator 2210 andexternal-transfer optical waveguides 2230/2231. High efficiency for eachtransfer of optical power yields a modulator having low insertion loss.Manufacturing and fabrication economies and wafer-scale pre-testing andpre-qualification capabilities described hereinabove would be realizedfor any device fabricated according to the present invention.

A variety of waveguide types may be employed as an external-transferoptical waveguide (adapted for adiabatic or mode-interference-coupledtransverse-transfer) while remaining within the scope of the presentinvention. A low-index dielectric waveguide including a core andlower-index cladding layers may be a preferred external-transferwaveguide. Such waveguides may be fabricated using silica,germanosilicate, other doped silicas, silicon nitride, siliconoxynitride, other glasses, polymers, and so forth using precisionmaterial processing techniques. The resulting waveguide may be readilymodal-index-matched to planar waveguides fabricated using similarmaterials or a silica-based fiber-optic-taper segment. Other waveguidematerials and/or configurations may be equivalently employed, andvarying waveguide properties exploited to modify and/or enhance thefunctionality of the optical device. A suitable external-transferoptical waveguide for a preferred embodiment of the present inventionmay: 1) be adapted at an end thereof for optical power end-transferbetween the external-transfer optical waveguide and an optical device;2) be adapted for optical power transverse-transfer between theexternal-transfer optical waveguide and an optical device (adiabatic,active modal-index-matched, or passive modal-index-matched); 3) beadapted at an optical junction segment thereof so as to yield suitablevalues of κ, L, and/or Δβ with a transmission optical waveguide formode-interference-coupled transverse-transfer (active or passivemodal-index-matched); and/or 4) be adapted at an optical junctionsegment thereof for substantially adiabatic transverse-transfer with thetransmission optical waveguide. Waveguides may be employed whereinconfinement of waveguide optical modes is effected by: one or morewaveguide cores surrounded by lower-index cladding layers, distributedBragg reflectors, other multi-layer reflectors, photoniccrystal/photonic bandgap techniques, metal reflector coatings,dielectric reflector coatings, and/or internal reflection at anair/waveguide interface. A waveguide with a core may include a singlecore or multiple-cores, the latter which may be employed for alteringthe field distribution of a supported isolated or system mode (so-called“field-flattening”, thereby further reducing alignment sensitivity of anassembled optical device). Waveguide materials may include (but are notlimited to) one or more of: silica, germanosilicate, and/or othersilica-based materials, silicon nitride, silicon oxynitride, organicmaterials, inorganic materials, crystalline materials, glassy oramorphous materials, polymeric materials, semiconductor materials,electro-optic materials, and so forth. Low-index dielectric waveguides(silica-based, for example) may be passively modal-index-matched to atransmission waveguide of similar index. Such waveguides may exhibitlittle or no dependence of transverse-coupling efficiency on wavelengthor polarization, and this may be desirable for a variety of opticaldevices employing external-transfer optical waveguides according to thepresent invention.

It may be desirable (particularly when employingmode-interference-coupled transverse-transfer) to modify the distal endof the external-transfer optical waveguide (the end that is not coupledto the optical device) in a variety of ways. It may be preferable tosubstantially eliminate optical feedback to the optical device arisingfrom optical power back-reflected from the distal end of theexternal-transfer optical waveguide. The distal end may be modified tosubstantially eliminate such feedback by providing an optical losselement (an optical absorber and/or optical scatterer), and/or byintentionally fabricating a mis-aligned end face of theexternal-transfer optical waveguide so that back-reflected optical powerdoes not propagate back through the waveguide. Such a canted end-facemay provide additional utility for testing and/or monitoring the opticaldevice. A substantially planar (or suitably curved, if focusingproperties are desired) canted or beveled end-face may serve to reflectoptical power transversely out of the external-transfer opticalwaveguide (down toward the substrate, substantially parallel to thesubstrate, or up away from the substrate). Optical power directed out ofthe external-transfer optical waveguide in this way may be detectedand/or analyzed for testing/characterization/monitoring of the opticaldevice. For a waveguide end-face canted or beveled to direct opticalpower up and away from the wafer, an external detector may be employedfor device testing and qualification during the manufacturing process.Alternatively, detectors may be integrated into/onto the wafer alongwith the optical devices and external-transfer optical waveguides, andmay remain as part of the finished optical devices to serve as in situmonitors of device performance in addition to enabling testing duringmanufacturing.

It may be desirable to provide wider tolerances for modal-index-matchingnear a given wavelength. In an alternative embodiment shown in FIG. 23,external-transfer optical waveguide 2330 may be fabricated with multipledistinct optical junction segments 2334 a-2334 d, each designed tomodal-index-match transmission optical waveguide 2320 at differingwavelengths. Preferably, the modal-index-matched wavelength variesmonotonically from segment 2334 a to 2334 d, and the respectivemodal-index-matched bandwidths for segments 2334 a-d should preferablyoverlap somewhat to provide substantially continuous wavelengthcoverage. During assembly of an optical device, transmission waveguide2320 may be sequentially coupled to each of segments 2334 a-d until asubstantially modal-index-matched segment is found. Four segments 2334are shown, but any suitable number of separate optical junction segments2334 may be provided.

Under certain circumstances it may be desirable to provide wavelength-or polarization-dependent transfer of optical power to/from an opticaldevice. For example, the device of FIG. 24 shows multiple semiconductorlaser sources 2410 a-d each coupled to a single transmission waveguide2420 through corresponding optically integrated external-transferoptical waveguides 2430 a-d to form a wavelength-mulitplexer. Lasers2410 a-d and external-transfer optical waveguides 2430 a-d arepreferably fabricated on a common substrate 2402. However, unless thetransverse-transfer between the transmission waveguide 2420 and eachexternal-transfer optical waveguide 2430 a-d is wavelength dependent,optical power transferred to the transmission waveguide 2420 from thefirst laser 2410 a will be at least partially transferred from thetransmission waveguide 2420 into subsequent external-transfer opticalwaveguides 2430 b-d and be lost. By providing wavelength-specifictransverse-transfer between the transmission waveguide 2420 and theoptical junction segments 2434 a-d of external-transfer opticalwaveguides 2430 a-d, optical power transferred to the transmissionwaveguide from one laser will pass subsequent external-transfer opticalwaveguides substantially undisturbed.

Wavelength-dependent transverse optical coupling may be most readilyachieved by manipulating modal-index-matching between the transmissionwaveguide and the external-transfer optical waveguide adapted formode-interference-coupled transverse-transfer. Dispersive properties andthe transverse size and/or shape of the optical junction segment of theexternal-transfer optical waveguide may be exploited to yieldtransverse-transfer only over a desired wavelength range. Materialdispersion alone may not be sufficient to yield a suitably narrowwavelength range for transverse-transfer. Multi-layer reflectorwaveguides (referred to as DBR waveguides in applications A1 and A2, asMLR waveguides in application A10) typically exhibit significantlygreater wavelength dispersion in the modal index of supported opticalmodes, and may be preferred for implementing wavelength-dependentexternal-transfer optical waveguides for optical devices according tothe present invention. Such waveguides are described in detail inearlier-cited applications A1, A2, and A10, and typically comprise acore layer between upper and lower λ/4 stacks of materials havingdiffering refractive indices (these stacks are also referred to asmulti-layer mirrors or multi-layer reflectors). The upper and lowerstacks may be the same or may differ in materials and/or number oflayers, depending on the desired waveguide characteristics (in someimplementations the upper stack and even the core may be missingentirely). Transverse-transfer to a MLR waveguide may be achieved fromthe side of the waveguide (substantially perpendicular to the MLR stackgrating wavevector) or from the surface of the waveguide (substantiallyalong the MLR stack grating wavevector). The stacks serve to confine thewaveguide optical modes, and give rise to the dispersive properties ofthe MLR waveguide. Suitable MLR waveguides for use in the presentinvention may be fabricated using dielectric and/or semiconductorlayers, and may be designed to exhibit the desired dispersiveproperties.

Sufficiently accurate design and fabrication of such MLR waveguides mayenable passively-modal-index-matched mode-interference-coupledtransverse-transfer between the transmission optical waveguide andexternal-transfer optical waveguide over a desired wavelength range.Alternatively, electro-optic properties of the MLR waveguide (eitherinherent in the materials used or specifically incorporated into one ormore electro-optic material layers) may be employed to enableactively-modal-index-matched mode-interference-coupledtransverse-transfer. As shown in FIG. 25, control electrodes or contacts2536 and 2538 may be provided on the optical junction segment 2534 ofthe external-transfer optical waveguide 2530, and a control voltageapplied to achieve modal-index-matching with the optical junctionsegment 2524 of fiber-optic taper 2520 over the desired wavelengthrange. Device 2510 and external-transfer optical waveguide 2530 maypreferably be fabricated on common substrate 2502 to achievesubstantially spatial-mode-matched end-coupling therebetween. Such anactively modal-index-matched implementation enables selection of adesired transverse-transfer wavelength even if the MLR waveguide cannotbe sufficiently accurately designed and fabricated, or if there aremanufacturing tolerances for the design wavelength of device 2510. Suchactive modal-index-matching also enables construction of optical deviceshaving dynamically re-configurable wavelength-dependent properties.

Alternatively, a multi-layer dispersion-engineered optical waveguide maybe employed as an external-transfer optical waveguide adapted foradiabatic optical power transverse-transfer. Optical properties of themulti-layer reflector waveguide may be varied along the length thereofin a variety of ways to achieve adiabatic transverse-transfer. Variationof refractive index, thickness, and width of one or more layers may beemployed for providing an external-transfer optical waveguide adaptedfor adiabatic transverse-transfer according to the present invention.This multi-layer reflector implementation may enjoy the relaxedalignment tolerances typical of adiabatic transverse-transfer, whilestill enabling active control over optical power transverse-transfer.

MLR waveguides typically exhibit polarization-dependent modal indicesfor supported optical modes. This property may be most readily exploitedto enable polarization-selective mode-interference-coupledtransverse-transfer between the transmission optical waveguide and theexternal-transfer optical waveguide. Such polarization-selectivetransverse-transfer may be desirable in a variety of circumstances,including polarization-dependent beam combining for delivering pumplaser power to doped-fiber gain media, among other examples. A varietyof polarization-dependent apparatus and methods for optical powertransverse-transfer are disclosed in earlier-cited application A4, alongwith various circumstances in which polarization-dependenttransverse-transfer may be employed to advantage.

It should be noted that, in addition to being used as anexternal-transfer optical waveguide according to the present invention,MLR waveguides may also be used as all or part of an optical device suchas a modulator, filter, N×N switch, multiplexer/demultiplexer, and soon. Optical devices thus implemented may be provided with one or moreexternal-transfer optical waveguides according to the present invention,and these external-transfer optical waveguides may include any of theexternal-transfer optical waveguide types disclosed herein, includingMLR waveguides. The external-transfer optical waveguides may be activelyor passively modal-index-matched for mode-interference-coupledtransverse-transfer to/from the transmission optical waveguide, and mayor may not exhibit wavelength- and/or polarization-dependenttransverse-coupling to the transmission waveguide. Alternatively,external-transfer optical waveguides may be implemented for adiabatictransverse-transfer according to the present invention. It should benoted that for optical devices according to the present inventionemploying a MLR waveguide for both the device portion as well as theexternal-transfer optical waveguide, each may comprise substantially thesame type of MLR structure, or each may comprise a distinct type of MLRstructure.

Examples of MLR-based optical devices with an optically integratedexternal-transfer optical waveguide according to the present inventionare shown in FIGS. 26A/26B and 27A/27B, each of which illustrate asingle port device based on a MLR waveguide and incorporating anexternal-transfer optical waveguide according to the present invention.In FIGS. 26A and 26B, MLR device 2610 is end-coupled toexternal-transfer optical waveguide 2630 (both fabricated on substrate2602), which is in turn adapted for transverse-transfer to/fromtransmission optical waveguide 2620, in these examples a fiber-optictaper segment. In the example of FIG. 26A, external-transfer opticalwaveguide 2630 is shown surface-joined to the fiber-optic taper segmentand adapted for mode-interference-coupled transverse-transfer withactive modal-index-matching provided by contacts 2632/2634. In theexample of FIG. 26B, external-transfer optical waveguide 2630 is shownside-joined to the fiber-optic taper segment and adapted formode-interference-coupled transverse-transfer with passivemodal-index-matching. Contacts 2612/2614 provide electronic access todevice 2610 (examples: to provide drive current for a laser; to providebias voltage and/or signal output for a detector; to provide a drivesignal for a modulator; and so on). External-transfer optical waveguide2630 may serve to transfer optical signal power between MLR device 2610and transmission optical waveguide 2620 in either or both directions asneeded.

In FIGS. 27A and 27B, MLR device 2710 is end-coupled toexternal-transfer optical waveguide 2730 (both fabricated on substrate2702), which is in turn adapted for transverse-transfer to/fromtransmission optical waveguide 2720, in these examples a planarwaveguide (waveguide substrate omitted from the Figures for clarity). Inthe example of FIG. 27A, external-transfer optical waveguide 2730 isshown surface-joined to the planar waveguide and adapted for adiabatictransverse-transfer therebetween. In the example of FIG. 27B,external-transfer optical waveguide 2730 is shown side-joined to theplanar waveguide and adapted for adiabatic transverse-transfer withactive control of the adiabatic condition provided by contacts2732/2734. Contacts 2712/2714 provide electronic access to device 2710

Many other device combinations and/or configuration may be implementedwhile remaining within the scope of inventive concepts disclosed herein,and examples are shown in FIG. 28A/B and subsequent Figures. The deviceconfigurations shown are exemplary and do not represent an exhaustiveset of device configurations that may be implemented according to thepresent invention. These devices may include any suitable device type orconstruction (including MLR-based devices) and may employ any suitableexternal-transfer optical waveguide type. These devices withexternal-transfer waveguides may be adapted for forming opticaljunctions with any suitable transmission optical waveguide types,including but not limited to fiber-optic taper segments and planarwaveguides. Optical power transfer between external-transfer opticalwaveguides and transmission optical waveguides may employmode-interference-coupled and/or adiabatic transverse-transfer, asdesired for a particular situation. An optical device with one or moreoptically integrated external-transfer optical waveguides may beadvantageously implemented in a flip-chip geometry (as in FIG. 15A/15B),particularly when intended to be used with planar transmission opticalwaveguides.

FIGS. 28A and 28B show an exemplary two-port optical device 2810optically integrated with external-transfer optical waveguides 2830/2831on substrate 2802 according to the present invention. The transmissionoptical waveguide 2820 in these examples is a fiber-optic taper segment.Electrodes or contacts 2812/2814 provide electronic access to device2810. In the example of FIG. 28A, optical power transverse-transferbetween external-transfer optical waveguides 2830/2831 andsurface-joined transmission optical waveguide 2820 is activemodal-index-matched mode-interference-coupled, with contacts2832/2833/2834/2835 provided for modal-index-matching. In the example ofFIG. 28B, optical power transverse-transfer between external-transferoptical waveguides 2830/2831 and side-joined transmission opticalwaveguide 2820 is passive modal-index-matched mode-interference-coupled.The devices of FIG. 28A/28B may be operated with substantially completetransfer of optical power from the transmission waveguide 2820 to thedevice through one of the external-transfer waveguides 2830/2831 andback to the transmission waveguide through the other external-transferwaveguide after manipulation by device 2810. Alternatively, device 2810and the segment of transmission waveguide 2820 between the twoexternal-transfer waveguides may function as the two arms of aMach-Zender interferometer. Device 2810 may function as a phasemodulator for one arm of the interferometer thus formed, therebyenabling modulation of transmission of optical power though transmissionwaveguide 2820 past external-transfer waveguides 2830/2831.

FIGS. 29A through 29D show Mach-Zender interferometer modulators 2910fabricated on substrate 2902 along with external-transfer opticalwaveguides 2930/2931. Optical power may be substantially completelytransferred from transmission waveguide 2920 into external-transferwaveguide 2930 and into Mach-Zender interferometer 2910. Contacts orelectrodes 2911/2912/2913/2914 are employed to control transmissionthrough interferometer 2910, and optical power transmitted therethroughis transferred into transmission waveguide 2920 throughexternal-transfer waveguide 2931. Mach-Zender interferometer 2910 may beconstructed using waveguides of any suitable type (including multi-layerwaveguides) incorporating electro-active materials of any suitable type.Instead of electrodes and electro-active materials, nonlinear opticalmaterials may be employed in Mach-Zender interferometer 2910 and opticalcontrol signals used to control transmission therethrough. FIG. 29Ashows transmission waveguide 2920 as a fiber-optic taper segmentsurface-joined to external-transfer waveguides 2930/2931 and employingpassive modal-index-matched mode-interference-coupledtransverse-transfer. FIG. 29B shows transmission waveguide 2920 as afiber-optic taper segment side-joined to external-transfer waveguides2930/2931 and employing active modal-index-matchedmode-interference-coupled transverse-transfer controlled by electrodes2932/2933/2934/2935. FIG. 29C shows transmission waveguides 2920/2921 asplanar waveguides (waveguide substrate omitted from FIG. 29C forclarity) surface-joined to external-transfer waveguides 2930/2931 andemploying adiabatic transverse-transfer. FIG. 29D shows transmissionwaveguides 2920/2921 as planar waveguides (waveguide substrate omittedfrom FIG. 29D for clarity) side-joined to external-transfer waveguides2930/2931 and employing adiabatic transverse-transfer.

FIG. 30 illustrates a 2×2 optical switch 3010 fabricated on substrate3002 along with external-transfer waveguides 3030/3031/3032/3033.Corresponding transmission waveguides 3020/3021/3022/3023 are shownsurface-joined to the external-transfer waveguides, and are shown asplanar optical waveguides (waveguide substrate omitted from FIG. 30 forclarity). Other transmission waveguide types (including fiber-optictaper segments) may be equivalently employed, and side-joining may beequivalently employed. Transverse-transfer of optical power between eachtransmission waveguide and the corresponding external-transfer waveguidemay be adiabatic, active-modal-index-matched mode-interference-coupled,or passive-modal-index-matched mode-interference-coupled. The nature ofthe transverse-transfer need not be the same for all transmissionwaveguide/external-transfer waveguide pairs. Control signals applied toelectrodes or contacts 3011/3012/3013/3014 control optical transmissionthrough the 2×2 switch 3010, which may be constructed using waveguidesof any suitable type (including multi-layer waveguides) incorporatingelectro-active materials of any suitable type. Instead of electrodes andelectro-active materials, nonlinear optical materials may be employed in2×2 switch 3010 and optical control signals used to control transmissiontherethrough.

FIG. 31 illustrates a resonant optical modulator including resonator3110, optical loss element 3117, and waveguide 3118 all fabricated onsubstrate 3102 along with external-transfer waveguides 3130/3131.Corresponding transmission waveguides 3120/3121 are shown surface-joinedto the external-transfer waveguides, and are shown as planar opticalwaveguides (waveguide substrate omitted from FIG. 31 for clarity). Othertransmission waveguide types (including fiber-optic taper segments) maybe equivalently employed, and side-joining may be equivalently employed.Transverse-transfer of optical power between each transmission waveguideand the corresponding external-transfer waveguide may be adiabatic,active-modal-index-matched mode-interference-coupled, orpassive-modal-index-matched mode-interference-coupled. The nature of thetransverse-transfer need not be the same for both transmissionwaveguide/external-transfer waveguide pairs. Control signals applied toelectrodes or contacts 3111/3112 (waveguide/resonator coupling),3113/3114 (resonator loss), and 3115/3116 (resonator frequency) controloptical transmission through the resonant modulator, which may beconstructed using waveguides of any suitable type (including multi-layerwaveguides) incorporating electro-active materials of any suitable type.Instead of electrodes and electro-active materials, nonlinear opticalmaterials may be employed in the resonant modulator and optical controlsignals used to control transmission therethrough. The device of FIG. 31may be employed to modulate only a specific band of wavelengthstransmitted through transmission waveguides 3120/3121 and waveguide3118.

FIG. 32 illustrates a resonant optical filter including resonator 3210and waveguides 3217/3218 all fabricated on substrate 3202 along withexternal-transfer waveguides 3230/3231/3232/3233. Correspondingtransmission waveguides 3220/3221/3222/3223 are shown surface-joined tothe external-transfer waveguides, and are shown as planar opticalwaveguides (waveguide substrate omitted from FIG. 32 for clarity). Othertransmission waveguide types (including fiber-optic taper segments) maybe equivalently employed, and side-joining may be equivalently employed.Transverse-transfer of optical power between each transmission waveguideand the corresponding external-transfer waveguide may be adiabatic,active-modal-index-matched mode-interference-coupled, orpassive-modal-index-matched mode-interference-coupled. The nature of thetransverse-transfer need not be the same for both transmissionwaveguide/external-transfer waveguide pairs. Control signals applied toelectrodes or contacts 3111/3112 (waveguide/resonator coupling),3113/3114 (waveguide/resonator coupling), and 3115/3116 (resonatorfrequency) control optical power transmission through the resonantfilter, which may be constructed using waveguides of any suitable type(including multi-layer waveguides) incorporating electro-activematerials of any suitable type. Instead of electrodes and electro-activematerials, nonlinear optical materials may be employed in the resonantmodulator and optical control signals used to control transmissiontherethrough. The device of FIG. 32 may be employed to switch only aspecific band of wavelengths between transmission waveguides 3217/3218.

In the examples of FIGS. 28A/28B and 29A/29B, the transmission opticalwaveguide is shown as a single waveguide adapted and positioned fortransverse-transfer to both external-transfer optical waveguides. In anydevice configuration wherein complete transfer of optical power from thetransmission waveguide is desired for subsequent manipulation by theoptical device and transfer back into the same transmission waveguide,it may be desirable to provide an optical loss mechanism on theintermediate portion of the single transmission waveguide. Such a lossmechanism may take the form of an additional optical waveguidepositioned between the external-transfer waveguides and adapted fortransverse-transfer, an absorbing or scattering coating, an absorbing orscattering transverse-coupled optical element, an absorbing orscattering structural element, a Bragg grating, doping, or other opticalloss mechanism. In this way the two ends of the transmission waveguidewould be de-coupled optically, while being mechanically coupled forfacilitating device fabrication and/or assembly. FIG. 33A shows anexample of a two-port device 3310 fabricated on a substrate 3302 withexternal-transfer optical waveguides 3330/3331 with transmissionwaveguide 3320 positioned and adapted for transverse-transfer with theexternal-transfer waveguides. An additional optical waveguide 3380provides optical loss by transverse-transfer from transmission waveguide3320 between the external-transfer waveguides. FIG. 33B shows similarexample in which a structural transmission-waveguide-alignment member3370 also provides optical loss between the external-transferwaveguides.

Most examples disclosed to this point have employed planar opticalwaveguides or fiber-optic taper segments as transmission waveguides.Side-etched fiber-optic segments as disclosed in earlier-citedapplication A6 may also be employed in conjunction with a suitablyarranged segment of an external-transfer waveguide fortransverse-transfer. Similarly, any optical waveguide that may besuitably configured for transverse-transfer adiabatic ormode-interference-coupled) may be employed for implementing the presentinvention.

Planar waveguides and planar waveguide circuits comprise an importantclass of transmission optical waveguides used to implement the presentinvention. A planar waveguide may often comprise a low-indexcore/cladding-type dielectric waveguide fabricated on a substantiallyplanar substrate, often silica or silica-based waveguides fabricated onan oxide-coated silicon substrate. Silicon is a desirable waveguidesubstrate material for a variety of reasons, including but not limitedto: relatively easy and well-understood material processing techniques;mature industry standards; ability to exploit economies of scale throughlarge wafer sizes, highly planar single crystal faces obtainable;amenable to selective dry- and/or wet-etching; highly rigid; desirablethermal characteristics. The silicon substrate is often provided with asilica over-layer, with one or more planar waveguides on the silicaover-layer. Silica and silica-based materials are nearly ideal andwell-understood optical materials. Alternatively, planar waveguides mayinstead comprise one or more high-index waveguides (semiconductorwaveguides, for example) formed on a quartz, silica, or other low-indexor insulating substrate (or over-layer on a semiconductor substrate).Such high-index waveguides may offer the advantage of more readilyachieved transverse-transfer to other high-index optical components.Silica-based planar waveguides have previously been used in end-coupledconfigurations with optical devices and/or optical fibers, but typicallyexhibit high insertion losses due to poor spatial-mode-matching,particularly with semiconductor-based optical devices. External-transferwaveguides implemented according to the present invention may offersignificant reduction in insertion losses for such optical devices byenabling transverse-transfer to/from the planar waveguide. An examplehas already been shown in FIGS. 13A/13B, 14A/14B, and 15A/15B adaptedfor adiabatic transverse-transfer between planar transmission waveguidesand external-transfer waveguides optically integrated with a two-portoptical device.

Another example is shown in FIGS. 34A and 34B including a two-portoptical devices 3410 (an electro-absorption modulator, for example) withoptically integrated input and output external-transfer waveguides3430/3431. The external-transfer waveguides may be of any type suitablefor spatial-mode-matched end-coupling to an optical mode defined by theoptical device and for transverse-transfer to the planar transmissionwaveguides 3420/3421. The transverse-transfer may be: adiabatic,active-modal-index-matched mode-interference-coupled, orpassive-modal-index-matched mode-interference-coupled. As withfiber-optic-taper-based transmission waveguides, the degree of alignmentprecision required for achieving efficient transverse-transfer betweenthe planar transmission waveguides and the external-transfer waveguidesmay be as much as an order-of-magnitude less than the precision requiredfor end-transfer between spatial-mode-matched planar transmissionwaveguide and optical device, thereby enabling passive alignmenttechniques for positioning the optical device relative to the planartransmission waveguide. In the exemplary embodiment of FIG. 34A/34B,optical device 3410 and external-transfer waveguides 3430/3431 arefabricated on a common substrate 3402. Planar transmission waveguides3430/3421 are fabricated on substrate 3422. The two components areassembled in a so-called “flip-chip” geometry in order to establishtransverse-transfer between external-transfer waveguides 3430/3431 andplanar transmission waveguides 3420/3421, and electronic couplingbetween electrode 3414 and contact 3424. An optical signal to bemanipulated may be received from an optical system through a planartransmission waveguide 3420, transferred by transverse-transfer intoexternal-transfer waveguide 3430, transferred by end-transfer intodevice 3410, manipulated as it propagates through the device 3410,transferred by end-transfer into the other external-transfer waveguide3431, transferred by transverse-transfer into the other planartransmission waveguide 3421, and transmitted to the optical system. Highefficiency for each transfer of optical power yields a device having lowinsertion loss. This type of planar waveguide implementation may beapplied with any suitable optical device.

Planar waveguide implementations of the present invention offer thepossibility of high levels of integration of multiple optical componentsto form hybrid or composite optical devices. Multiple planartransmission waveguides forming an optical network of any desiredtopology may be fabricated on a substrate with a gap at each point wherean optical component might be located. The transmission planarwaveguides may be adapted near each potential device location foroptical power transverse-transfer (adiabatic and/ormode-interference-coupled; both types may be mixed on a single planarwaveguide substrate). Each optical component may be an optical devicewith one or more optically integrated external-transfer waveguidesaccording to the present invention, each positioned relative to thecomponent device for efficient end- or transverse-transfer to/from theoptical device. Each external-transfer waveguide may be positioned andadapted so as to enable transverse-transfer to/from a correspondingplanar transmission waveguide when the component is positioned on theplanar waveguide substrate. A so-called “flip chip” geometry may beemployed to establish transverse-transfer between each external-transferwaveguide and its corresponding planar waveguide(s) on the substrate.Mating alignment structures may be provided on the component and/or onthe planar waveguide substrate for establishing suitably preciserelative positioning of the external-transfer waveguides and therespective planar transmission waveguides.

Examples of an optical device with multiple transverse-coupledsub-components are shown in FIGS. 35A/35B, 36A/36B, and 37A/37B, inwhich a laser/waveguide hybrid component and a modulator/waveguidehybrid component, each having one or more optically integratedexternal-transfer waveguides, are flip-chip mounted onto a planarwaveguide substrate 3522. One planar transmission waveguide 3524transmits optical power from the laser 3542 to the modulator 3552, whilea second planar transmission waveguide 3526 may terminate in atransverse-transfer segment 3528 for transverse-transfer of modulatedoptical power to a fiber-optic taper 3520 (FIG. 35A). Optical power fromthe laser 3542 is: transferred by end-transfer into an external-transferwaveguide 3546 (optically integrated with laser 3542 on substrate 3544);transferred by transverse-transfer to the first planar transmissionwaveguide 3524; transferred by transverse-transfer to an inputexternal-transfer waveguide 3556 (optically integrated with modulator3552 on substrate 3554); transferred by end-transfer to the modulator3552; modulated as it propagates through the modulator 3552; transferredby end-transfer to an output external-transfer waveguide 3557 (opticallyintegrated with modulator 3552 on substrate 3554); transferred bytransverse-transfer to the second planar transmission waveguide 3526;transferred by transverse-transfer to the fiber-taper-segment 3520 ofthe optical fiber (FIG. 35A). The transverse-transfer steps may beadiabatic (FIG. 36A/36B) or mode-interference-coupled (active orpassive; FIG. 37A/37B), and need not be of the same type for eachtransverse-transfer optical junction in the assembly.

Instead of transverse-transfer to a fiber-optic taper, transmissionwaveguide 3526 may instead be adapted to serve as a spatial-modeexpander for end-transfer to an optical fiber 3529 (FIG. 35B; seediscussion below) or other large-mode optical waveguide. High overalloptical power throughput (i.e., low insertion loss) may be achieved forthe hybrid device. In a manner similar to that shown in FIG. 35A, thewavelength multiplexed example of FIG. 24 may be constructed usingplanar transmission waveguides on a substrate for optical powertransmission to a fiber-optic taper, while each laser source comprises aflip-chip mounted laser having an integrated external-transfer waveguidefor transverse-transfer to the planar transmission waveguides. Anyoptical device may be coupled to one or more planar transmissionwaveguides as described hereinabove using one or more external-transferwaveguides according to the present invention. While a fiber-optic tapersegment 3520 is shown in FIG. 35A for transferring optical power to/fromthe planar waveguide substrate by transverse-transfer from/to planartransmission waveguide 3528, other methods and/or configurations may beequivalently employed. Planar transmission optical waveguide 3526 mayinclude a spatial-mode expander segment so that the propagating opticalpower is end-transferred to an optical fiber 3529. The spatial-modeexpansion should preferably be substantially adiabatic to minimizeundesirable optical loss.

Apparatus and methods according to the present invention may be furtherapplied to enable an optical “breadboard” approach for assemblingcomplex optical devices. A substrate may be provided with a plurality oflocations provided for flip-chip mounting of modular optical devices,each of the devices incorporating one or more external-transferwaveguides according to the present invention. These flip-chip mountinglocations may be provided on the substrate in an array pattern (square,rectangular, trigonal/hexagonal, or other) and a plurality of planartransmission waveguides may be provided on the substrate connecting eachflip-chip mounting location to one or more of its neighbors. Individualmodular optical devices of any desired type may be provided withexternal-transfer waveguide(s) positioned so as to establishtransverse-transfer to/from corresponding planar transmissionwaveguide(s) when the device is flip-chip mounted at a mounting locationon the substrate. Additional flip-chip components may be provided havingonly an external-transfer waveguide thereon (with no additional device)for optically “bridging” an otherwise vacant flip-chip mounting location(a single external-transfer waveguide would establishtransverse-transfer with two of the planar transmission waveguides). Anydesired combination of these modular optical devices may then beoptically coupled in virtually any combination and in virtually anytopology to construct complex hybrid optical devices. The opticalbreadboard may be further provided with planar waveguides adapted fortransverse-transfer to fiber-optic-taper segments or other waveguides,or end-transfer to an optical fiber or other waveguide, thereby enablingtransfer of optical power to and/or from the breadboard device. Such amodular breadboard approach may be useful for device prototyping and/orfor flexible device manufacture.

Alternatively, methods and apparatus according to the present inventionmay be employed for even higher degrees of optical device integration. Asingle planar waveguide substrate with multiple planar waveguides,multiple optical junction segments, and multiple device locations may befabricated in any desired planar waveguide circuit topology. Multipledevices with multiple corresponding external-transfer waveguides may befabricated on a single device substrate. The multiple devices andexternal-transfer optical waveguides may be positioned on the devicesubstrate in positions corresponding to the arrangement of the devicelocations on the waveguide substrate. Similarly, the optical junctionregions of the multiple external-transfer waveguides may be positionedto correspond to the optical junction regions of the planar waveguides.A single assembly step, assembling the waveguide substrate and thedevice substrate, may then serve to simultaneously establish opticaljunctions between the multiple planar waveguides and the multipleexternal-transfer waveguides to form a composite optical device of anydesired degree of complexity. A majority of the precise alignmentrequired may be accomplished during fabrication of the substrates andstructures thereon using precision, highly parallel material processingtechniques.

Optical devices and optically integrated external-transfer waveguidesmay be fabricated on a common substrate for modifying and/or controllingdevice performance or characteristics or otherwise providing devicefunctionality. In the example of FIG. 38, a laser 3810 is fabricated ona substrate 3802 along with optically integrated external-transferwaveguides 3830 and 3831. Waveguide 3830 may provide optical powertransfer between laser 3810 and transmission waveguide 3820. Waveguide3831 may serve to alter the optical properties and/or performance oflaser 3810. For example, a grating structure in waveguide 3831 may serveto define a lasing wavelength for laser 3810. Alternatively, spatialoptical modes supported by waveguide 3831 may define transverse spatialmode characteristics of the laser output. External-transfer waveguidesof varying structure and characteristics may be employed according tothe present invention for providing various optical devicefunctionalities for a variety of optical devices, including but notlimited to: wavelength selectivity, spatial mode selectivity, opticalfiltering, polarization selectivity, thermal compensation, powermonitoring, reflectivity, modulation, and so on. Selectivity may referto tuning, stabilization, and/or modulation of the relevant opticalproperty. The external-transfer optical waveguide may include a gratingstructure, a thermo-optic element, or other suitable functionalcomponent. End facets of devices may be manipulated during fabricationand/or processing to alter optical characteristics of devicesimplemented according to the present invention. For example, the endfacets may be coated for wavelength specificity, polarizationspecificity, and so on. A device end facet may be fabricated with acurvature so as to act as a lens, thereby altering the propagationcharacteristics of optical input to and/or output from the device. Acanted or beveled end facet may serve to direct optical powertransversely out of the external-transfer waveguide (acting as a turningmirror) for device monitoring or to reduce optical feedback.

An alternative embodiment is shown in FIG. 39 wherein a portion of theoptical device functionality is provided in a transmission opticalwaveguide, and/or the optical device is not fully functional untiltransfer of optical power between the device and transmission waveguideis enabled (through an external-transfer optical waveguide). A planarwaveguide substrate 3922 is shown with planar transmission waveguides3920 and 3921. Laser diode 3910 is provided on substrate 3902 withintegral external-transfer optical waveguides 3930 and 3931.Transverse-transfer between external-transfer waveguide 3930 andtransmission waveguide 3920 provides an output optical path for theoutput of laser 3910. Transfer of optical power between laser 3910 andtransmission waveguide 3921 (through external-transfer waveguide 3931)may enable control, modification, and/or manipulation of the output oflaser 3910 by making transmission waveguide 3921 in effect part of thelaser cavity. Control or manipulation of optical/spectralcharacteristics of transmission waveguide 3921 would have acorresponding effect on the output of laser 3910. For example, a gratingstructure may be provided in transmission waveguide 3921 to stabilizethe wavelength of laser 3910, or the spatial-mode characteristics oftransmission waveguide 3921 may control the spatial-mode characteristicof the output of laser 3910. Other devices and schemes may be readilydevised while remaining within the scope of the present invention. Itmay be the case that a device may not become functional until opticalpower transfer is established between the device and a transmissionoptical waveguide providing a portion of the device functionality.Transmission waveguides of varying structure and characteristics may beemployed according to the present invention for providing variousoptical device functionalities for a variety of optical devices,including but not limited to: wavelength selectivity, spatial modeselectivity, optical filtering, polarization selectivity, thermalcompensation, power monitoring, reflectivity, modulation, and so on. Thetransmission optical waveguide may include a grating structure, athermo-optic element, or other suitable functional component. In someexamples the transmission waveguide may only modify the operation of theoptical device, while in other examples the transmission waveguide maybe required for the device to function at all.

In light of the discussion of the preceding two paragraphs, the term“optical device” may require some clarification. As used herein,“optical device” may denote an independently functioning component, suchas a laser, modulator, filter, switch, and so forth. Alternatively,“optical device” may also denote a component that may not operateindependently, but must be used in conjunction with another component tofunction. An example may comprise an semiconductor gain medium with ananti-reflection coated end facet and an external-transfer waveguide.Alone such a component may not function as a laser source. If areflector were provided in the external-transfer waveguide, or in atransmission waveguide forming an optical junction with theexternal-transfer waveguide, then the reflector and semiconductor mighttogether form a functioning laser. Many other similar examples may fallwithin the scope of inventive concepts disclosed and/or claimed herein.

FIGS. 40 and 41 are process flow diagrams (transverse cross section andplan views, respectively) showing fabrication of a preferred planartransmission waveguide according to the present invention. Dimensionsand material compositions are exemplary and may be altered whileremaining within the scope of the present invention. A silicon substrate4002 is prepared with a 5 μm thick buffer layer of silica 4004, a 10 μmthick layer of germanosilica 4006, and a 70 nm thick layer of siliconnitride 4008 (Si_(x)N_(y)). The silicon nitride is patterned and etchedto form a waveguide core 4124 about 6 μm wide and of the desiredgeometry, shown in FIG. 41 laterally tapered at each end. Thegermanosilica layer is then etched to form a ridge waveguide 4120 about10 μm wide and about 6 μm high, as well as the top portions ofalignment/support structures 4170. Then both the remaininggermano-silica 4006 and the silica buffer 4004 are patterned andremoved, leaving ridge waveguide 4120 supported by a somewhat widerbuffer ridge 4126 comprising the germano-silica layer (about 4 μmremaining thickness) and the silica layer. The lower portions ofalignment/support structures 4170 are also formed by these steps. Ridges4120 and 4126 are then covered with a 0.5 μm thick deposited overlayerof germano-silica. The underlying silicon is patterned and etched toform a v-groove 4150 for eventually receiving the end of a single-modeoptical fiber. A groove 4152 is provided (by saw-cut or any othersuitable method) to terminate the v-groove and allow an optical fiber toreach the end of waveguide 4120. Contacts/electrodes 4160 and otherdesired elements may then be provided using suitable spatially selectivematerial processing techniques. While patterning/etching is recited forvarious processing steps herein, any suitable spatially selectivematerial processing technique(s) may be equivalently employed. Similarprocessing sequences may be employed to produce various configurations,such as the exemplary embodiments shown in FIGS. 42A/42B, 43A/43B, and44A/44B.

These planar waveguide components (equivalently, PLC-like components)may then receive an optical device with one or more integralexternal-transfer waveguides according to the present invention,preferably in a flip-chip geometry or in any suitable assembly geometry.One tapered end of the silicon nitride core of the ridge waveguide 4120is adapted for adiabatic optical power transverse-transfer with anexternal-transfer optical waveguide of the optical device, with theridge waveguide 4120 serving as the transmission optical waveguide.Alternatively, the ridge waveguide 4120 may be configured with theexternal-transfer optical waveguide for mode-interference-coupledoptical power transverse-transfer. The other tapered end of the siliconnitride core of ridge waveguide 4120 serves as a mode expander forenabling end-transfer of optical power between the ridge waveguide 4120and an optical fiber positioned in v-groove 4150 (fiber not shown). Asthe width of core 4122 decreases, the optical mode supported bycore/waveguide 4122/4120 expands into waveguide 4120. Preferably,tapering of core 4122 should be sufficiently gradual so that the modeexpansion satisfies the adiabatic condition (as defined earlier herein).By selecting the appropriate transverse dimension for ridge waveguide4120, a desired degree of spatial-mode matching between waveguide 4120and the optical fiber may be attained for enabling optical powerend-transfer between waveguide 4120 and the optical fiber.

In FIGS. 42A and 42B, a planar waveguide substrate 4222 is shown with aplanar transmission waveguide 4220 thereon adapted at a first end foradiabatic optical power transverse-transfer with an external-transferoptical waveguide of a diode laser. The diode laser andexternal-transfer waveguide are optically integrated on laser substrate4202, shown flip-chip mounted onto substrate 4222 in FIG. 42B. Av-groove 4250 in substrate 4222 serves to position an optical fiber 4290(shown in FIG. 42B) for end-transfer with waveguide 4220. Waveguide 4220may be adapted at a second end for mode expansion and a degree ofspatial-mode matching with the fiber 4290. Substrate 4222 may beprovided with an auxiliary waveguide 4240 positioned and adapted at afirst end thereof for transfer of a fraction of the optical output powerof the diode laser from waveguide 4220. A second end of waveguide 4240may be adapted for delivering optical power to a monitor photodiode.Contacts/electrodes 4224/4244 are provided for electronic access to thediode laser and the photodiode, respectively. Alignment/support members4270/4272 are provided for alignment and support of the flip-chipmounted diode laser and photodiode, respectively.

The monitor photodiode may be integrated onto planar waveguide substrate4222, or may preferably be provided as a separate component on aphotodiode substrate 4242, shown flip-chip mounted onto planar waveguidesubstrate 4222 in FIG. 42B. The first end of waveguide 4240 maypreferably be adapted for transverse-transfer of optical power fromwaveguide 4220 (as shown in FIG. 42A/42B), or may alternatively beadapted in any other suitable way for optical power transfer fromwaveguide 4220. The second end of waveguide 4240 may preferably beadapted to function as a turning mirror for directing optical powerupward and away from substrate 4222 and onto the monitor photodiode.This may be accomplished during processing of substrate 4222 byproviding a beveled end-facet at the second end of waveguide 4222.Alternatively, any suitable means may be employed for delivering opticalsignal power from waveguide 4240 to the monitor photodiode. For example,the second end of waveguide 4240 may be provided with an opticalscatterer, with a portion of the scattered optical power detected by thephotodiode. In another example, the photodiode may be provided with anexternal-transfer optical waveguide according to the present inventionadapted for transverse-transfer (adiabatic or mode-interference-coupled)of optical power from waveguide 4240 and delivery of the optical powerto the photodiode. Other means may be equivalently employed.

FIG. 43A/43B show a planar waveguide substrate 4322 with planartransmission optical waveguides 4320 and 4321 thereon. A diode laser maybe provided with an external-transfer optical waveguide on lasersubstrate 4302, shown flip-chip mounted onto substrate 4322 in FIG. 43B.An optical modulator may be provided with two external-transfer opticalwaveguides on modulator substrate 4303, shown flip-chip mounted ontosubstrate 4322 in FIG. 43B. Waveguide 4320 may be adapted at a first endfor transverse-transfer of optical power (adiabatic ormode-interference-coupled) with the external-transfer waveguide of thediode laser, and may be adapted at a second end for transverse-transferof optical power (adiabatic or mode-interference-coupled) with a firstof the external-transfer waveguides of the modulator. Waveguide 4321 maybe adapted at a first end for transverse-transfer of optical power witha second of the external-transfer waveguides of the modulator (adiabaticor mode-interference-coupled), and adapted at a second end formode-expansion and end-transfer with an optical fiber 4390, shownpositioned in v-groove 4350 in FIG. 43B. Auxiliary waveguides 4340 and4341 are provided on substrate 4322 positioned and adapted at the firstends thereof for transfer of a fraction of the optical power fromwaveguides 4320 and 4321, respectively. The fractions diverted aredirected to photodiodes, preferably provided on separate photodiodesubstrates 4344/4345 and shown in FIG. 43B flip-chip mounted ontosubstrate 4322. Waveguides 4340/4341 and the photodiodes may be adaptedas described in the preceding paragraphs. Contacts/electrodes4370/4371/4372/4373 are provided for electronic access to the diodelaser, modulator, and photodiodes. Alignment/support members4380/4381/4382/4383 may be provided on substrate 4322 for alignment onsupport of the diode laser, modulator, and photodiodes. The foregoingembodiments are exemplary. Many other configurations of optical devices(with external-transfer waveguides) and transmission waveguides may fallwithin the scope of the present invention as disclosed and/or claimedherein.

In FIGS. 44A and 44B, a planar waveguide substrate 4422 is shown with aplanar transmission waveguide 4420 thereon adapted at a first end foradiabatic optical power transverse-transfer with an external-transferoptical waveguide of a diode laser. The diode laser andexternal-transfer waveguide are optically integrated on laser substrate4402, shown flip-chip mounted onto substrate 4422 in FIG. 44B. Av-groove 4450 in substrate 4422 serves to position an optical fiber 4490(shown in FIG. 44B) for end-transfer with waveguide 4420. Waveguide 4420may be adapted at a second end for mode expansion and a degree ofspatial-mode matching with the fiber 4290 in conjunction with ball lens4494, shown received in a recessed portion of substrate 4422. Substrate4422 may have an additional recessed portion for receiving an opticalisolator 4496 inserted between ball lens 4494 and optical fiber 4490.Substrate 4422 may be provided with an auxiliary waveguide 4440positioned and adapted at a first end thereof for transfer of a fractionof the optical output power of the diode laser from waveguide 4420. Asecond end of waveguide 4440 may be adapted for delivering optical powerto a monitor photodiode provided on flip-chip mounted substrate 4444.Contacts/electrodes 4470/4472 are provided for electronic access to thediode laser and the photodiode, respectively. Alignment/support members4480/4482 are provided for alignment and support of the flip-chipmounted diode laser and photodiode, respectively.

Various of the exemplary embodiments shown herein includesupport/alignment members for accurately positioning and supporting anoptical device (on a substrate with an external-transfer waveguide) on aplanar waveguide substrate. It may be desirable to provide supportand/or alignment structures on the device substrate as well. Suchsupport structures may serve to protect the external-transfer opticalwaveguide (often a protruding structure) from damage during assembly ofthe device and the waveguide substrate. Exemplary support members areshown in FIGS. 45A and 45B. An optical device (not shown) andexternal-transfer optical waveguide 4530 are optically integrated ondevice substrate 4502, along with support members 4560 (in the form ofelongated protruding ridges substantially parallel to waveguide 4530 inthe example; other configurations may be implemented). Upon assembly ofthe device with a planar transmission waveguide 4520 on waveguidesubstrate 4522 (omitted from FIG. 45A for clarity), the ridges 4560engage the surface of the waveguide substrate to provide mechanicalsupport and protection for external-transfer waveguide 4530.

For purposes of the present disclosure and appended claims, theconjunction “or” is to be construed inclusively (e.g., “a dog or a cat”would be interpreted as “a dog, or a cat, or both”; e.g., “a dog, a cat,or a mouse” would be interpreted as “a dog, or a cat, or a mouse, or anytwo, or all three”), unless: i) it is explicitly stated otherwise, e.g.,by use of “either . . . or”, “only one of . . . ”, or similar language;or ii) two or more of the listed alternatives are mutually exclusivewithin the particular context, in which case “or” would encompass onlythose combinations involving non-mutually-exclusive alternatives. Forpurposes of the present disclosure or appended claims, the words“comprising,” “including,” “having,” and variants thereof, wherever theyappear, shall be construed as open ended terminology, with the samemeaning as if the phrase “at least” were appended after each instancethereof. It is intended that equivalents of the disclosed exemplaryembodiments and methods shall fall within the scope of the presentdisclosure and/or appended claims. It is intended that the disclosedexemplary embodiments and methods, and equivalents thereof, may bemodified while remaining within the scope of the present disclosure orappended claims.

1. A method comprising: forming a first optical waveguide on a firstsubstrate; forming a second, structurally discrete optical waveguide ona structurally discrete second substrate; assembling the secondsubstrate or second optical waveguide with the first substrate or firstoptical waveguide so that the first and second optical waveguides arepositioned between the first and second substrates and are relativelypositioned for transferring the optical signal therebetween via opticaltransverse coupling; and arranging the first or second optical waveguidefor transferring the optical signal therebetween via substantiallyadiabatic optical transverse coupling with the first and secondwaveguides so positioned.
 2. The method of claim 1 further comprisingarranging the first or second optical waveguide for maintainingtransverse-offset optical power transfer loss therebetween less thanabout 0.5 dB for relative transverse offsets of the optical waveguidesless than about ±1.0 times a corresponding transverse optical mode sizecharacteristic of the optical waveguides.
 3. The method of claim 1further comprising arranging the first or second optical waveguide formaintaining transverse-offset optical power transfer loss therebetweenwithin about ±0.5 dB of a nominal optical power transfer loss level forrelative transverse offsets of the optical waveguides less than about±1.0 times a corresponding transverse optical mode size characteristicof the optical waveguides.
 4. The method of claim 1 wherein the first orsecond optical waveguide comprises a low-modal-index optical waveguide.5. The method of claim 1 further comprising structurally arranging thefirst or second substrate for positioning the first and second opticalwaveguides for establishing optical transverse coupling therebetween. 6.The method of claim 1 further comprising arranging the second opticalwaveguide at a distal end thereof for enabling optical end couplingbetween the second optical waveguide and an optical fiber mounted on thesecond substrate.
 7. The method of claim 6 further comprising mountingan optical fiber on the second substrate and end coupling the opticalfiber with the second optical waveguide at the distal end thereof. 8.The method of claim 6 wherein: the first optical waveguide comprises asilicon nitride or silicon oxynitride core and lower-index silica ordoped silica cladding; the second optical waveguide comprises a dopedsilica core and lower-index silica or doped silica cladding; and atleast one transverse dimension of the core or cladding of the first orsecond optical waveguide varies longitudinally along at least a portionof the optical junction region.
 9. The method of claim 1 wherein atleast a portion of the first or second optical waveguide comprises acore and lower-index cladding, and at least one transverse dimension ofthe core or the cladding varies longitudinally along at least a portionof the optical junction region.
 10. The method of claim 1 wherein: thefirst or second optical waveguide comprises a core and lower-indexcladding; the core comprises silicon nitride, silicon oxynitride, ordoped silica; and the cladding comprises silica or doped silica.
 11. Themethod of claim 1 further comprising securing together with a joiningelement the first substrate, first waveguide, second substrate, orsecond waveguide.
 12. The method of claim 11 wherein the joining elementcomprises a retainer, a clamp, a fastener, an adhesive, solder, pottingor embedding material, a clip, a tab and slot, or a spring ormicro-spring.
 13. The method of claim 1 wherein assembling the secondsubstrate or second optical waveguide with the first substrate or firstoptical waveguide includes arranging the first and second waveguides tobe relatively positioned within a transverse offset tolerance range. 14.The apparatus of claim 1 wherein assembling the second substrate orsecond optical waveguide with the first substrate or first opticalwaveguide includes positioning an exposed surface of the first opticalwaveguide against an exposed surface of the second optical waveguide.15. A method comprising: forming a first optical waveguide on a firstsubstrate, the first optical waveguide including a first opticaljunction region; forming a second optical waveguide on a secondsubstrate, wherein the second optical waveguide includes a secondoptical junction region, and the second substrate and the second opticalwaveguide are structurally discrete from the first substrate and thefirst optical waveguide; and arranging the first and second substrateswith the first and second optical junction regions disposedtherebetween, wherein the first and second optical junction regions havea relative position which enables substantially adiabatic opticaltransverse coupling of the optical signal therebetween.
 16. The methodof claim 15 wherein the relative position maintains a transverse-offsetoptical power transfer loss between the optical waveguides of less thanabout 0.5 dB for relative transverse offsets of the optical waveguidesless than about ±1.0 times a corresponding transverse optical mode sizecharacteristic of the optical waveguides.
 17. The method of claim 15wherein the relative position maintains a transverse-offset opticalpower transfer loss between the optical waveguides within about ±0.5 dBof a nominal optical power transfer loss level for relative transverseoffsets of the optical waveguides less than about ±1.0 times acorresponding transverse optical mode size characteristic of the opticalwaveguides.
 18. The method of claim 15 wherein the first or secondoptical waveguide comprises a low-modal-index optical waveguide.
 19. Themethod of claim 15 further comprising arranging the second opticalwaveguide at a distal end thereof for enabling optical end couplingbetween the second optical waveguide and an optical fiber mounted on thesecond substrate.
 20. The method of claim 19 further comprising mountingan optical fiber on the second substrate and end-coupling the opticalfiber with the second optical waveguide at the distal end thereof. 21.The method of claim 19 wherein: the first optical waveguide comprises asilicon nitride or silicon oxynitride core and lower-index silica ordoped silica cladding; the second optical waveguide comprises a dopedsilica core and lower-index silica or doped silica cladding; and atleast one transverse dimension of the core or cladding of the first orsecond optical waveguide varies longitudinally along at least a portionof the optical junction region.
 22. The method of claim 15 wherein atleast a portion of the first or second optical waveguide comprises acore and lower-index cladding, and at least one transverse dimension ofthe core or the cladding varies longitudinally along at least a portionof the respective optical junction region.
 23. The method of claim 15wherein: the first or second optical waveguide comprises a core andlower-index cladding; the core comprises silicon nitride, siliconoxynitride, or doped silica; and the cladding comprises silica or dopedsilica.
 24. The method of claim 15 further comprising securing the firstand second substrates together using a joining element, with the firstand second optical junction regions relatively positioned to enablesubstantially adiabatic optical transverse coupling.
 25. The method ofclaim 24 wherein the joining element comprises a retainer, a clamp, afastener, an adhesive, solder, potting or embedding material, a clip, atab and slot, or a spring or micro-spring.
 26. The method of claim 15wherein arranging the first and second substrates includes arranging thefirst and second substrates such that the first and second opticaljunction regions are relatively positioned within a transverse offsettolerance range.
 27. The apparatus of claim 15 wherein arranging thefirst and second substrates includes positioning an exposed surface ofthe first optical waveguide against an exposed surface of the secondoptical waveguide.
 28. A method comprising: forming a first opticalwaveguide on a first substrate to form a first device subunit, whereinthe first optical waveguide includes a first optical junction region;forming a second optical waveguide on a second substrate to form asecond device subunit, wherein the second optical waveguide includes asecond optical junction region, and the second device subunit isstructurally discrete from the first device subunit; assembling thefirst device subunit and the second device subunit into an opticaltransfer configuration, in which the first and second optical junctionregions are positioned facing each other between the first and seconddevice substrates, wherein the optical transfer configuration enablessubstantially adiabatic transverse-transfer of optical power between thefirst and second optical waveguides and includes a transverse offsettolerance range between respective positions of the first and secondoptical junction regions.
 29. The method of claim 28 wherein thetransverse offset tolerance range is characterized by maintainingtransverse-offset optical power transfer loss between the first andsecond optical waveguides of less than about 0.5 dB for relativetransverse offsets of the optical waveguides less than about ±1.0 timesa corresponding transverse optical mode size characteristic of theoptical waveguides.
 30. The method of claim 28 wherein the transverseoffset tolerance range is characterized by maintaining transverse-offsetoptical power transfer loss between the first and second opticalwaveguides of less than about ±0.5 dB for nominal optical power transferloss for relative transverse offsets of the optical waveguides less thanabout ±1.0 times a corresponding transverse optical mode sizecharacteristic of the optical waveguides.
 31. The method of claim 28wherein the first optical waveguide or the second optical waveguidecomprises a low-modal-index optical waveguide.
 32. The method of claim28 wherein the first optical waveguide or the second optical waveguidecomprises a silica-based optical waveguide.
 33. The method of claim 28wherein: the first optical waveguide comprises a core and lower-indexcladding; the core comprises silicon nitride, silicon oxynitride, ordoped silica; and the cladding comprises silica or doped silica.
 34. Themethod of claim 28 wherein at least a portion of the first opticalwaveguide comprises a core and lower-index cladding, and at least onetransverse dimension of the core or the cladding varies longitudinallyalong at least a portion of the first optical junction region.
 35. Themethod of claim 28 wherein the second optical waveguide is adapted at adistal end thereof for enabling end-transfer of optical power to anoptical fiber.
 36. The method of claim 35 further comprising mounting anoptical fiber on the second substrate and end coupling the optical fiberwith the second optical waveguide at the distal end thereof.
 37. Themethod of claim 35 wherein: the first optical waveguide comprises asilicon nitride or silicon oxynitride core and lower-index silica ordoped silica cladding; the second optical waveguide comprises a dopedsilica core and lower-index silica or doped silica cladding; and atleast one transverse dimension of the core or cladding of the first orsecond optical waveguide varies longitudinally along at least a portionof the optical junction region.
 38. The method of claim 28 furthercomprising securing the first and second device subunits together usinga joining element in the optical transfer configuration.
 39. The methodof claim 38 wherein the joining element comprises a retainer, a clamp, afastener, an adhesive, solder, potting or embedding material, a clip, atab and slot, or a spring or micro-spring.
 40. The apparatus of claim 28wherein assembling the first device subunit and the second devicesubunit includes positioning an exposed surface of the first opticalwaveguide against an exposed surface of the second optical waveguide.41. A method comprising: forming first device subunit that comprises afirst optical waveguide on a first substrate, which first opticalwaveguide includes a first optical junction region; adapting the firstdevice subunit for assembly with a structurally discrete second devicesubunit that comprises a second optical waveguide formed on a secondwaveguide substrate, which second optical waveguide includes a secondoptical junction region; adapting the first device subunit so thatassembly with the structurally discrete second device subunit positionsthe first and second optical junction regions between the first andsecond waveguide substrates; and adapting the first optical waveguide toenable substantially adiabatic transverse-transfer of optical powerbetween the first and second optical waveguides at the first and secondoptical junction regions.
 42. The method of claim 41 wherein the firstoptical waveguide comprises a silicon nitride, silicon oxynitride, ordoped silica core and lower-index silica or doped silica cladding. 43.The method of claim 41 wherein at least a portion of the first opticalwaveguide comprises a core and lower-index cladding, and at least onetransverse dimension of the core or the cladding varies longitudinallyalong at least a portion of the first optical junction region.
 44. Themethod of claim 41 further comprising arranging the first opticalwaveguide at a distal end thereof for enabling optical end couplingbetween the first optical waveguide and an optical fiber mounted on thefirst substrate.
 45. A method comprising assembling first and seconddevice subunits into an optical transfer configuration, wherein: thefirst device subunit comprises a first optical waveguide on a firstsubstrate and the first optical waveguide includes a first opticaljunction region; the second device subunit comprises a second opticalwaveguide on a second substrate, the second optical waveguide includes asecond optical junction region, and the second device subunit isstructurally discrete from the first device subunit; with the first andsecond device subunits in the optical transfer configuration, therespective first and second optical junction regions are positionedfacing each other between the first and second device substrates; andthe optical transfer configuration enables substantially adiabatictransverse-transfer of optical power between the first and secondoptical waveguides and includes a transverse offset tolerance rangebetween respective positions of the first and second optical junctionregions.
 46. The method of claim 45 wherein: the first optical waveguidecomprises a silicon nitride or silicon oxynitride core and lower-indexsilica or doped silica cladding; the second optical waveguide comprisesa doped silica core and lower-index silica or doped silica cladding; andat least one transverse dimension of the core or cladding of the firstor second optical waveguide varies longitudinally along at least aportion of the optical junction region.
 47. The method of claim 45further comprising mounting an optical fiber on the second substrate soas to end couple the optical fiber with the second optical waveguide ata distal end thereof.
 48. The method of claim 47 wherein: the firstoptical waveguide comprises a silicon nitride or silicon oxynitride coreand lower-index silica or doped silica cladding; the second opticalwaveguide comprises a doped silica core and lower-index silica or dopedsilica cladding; and at least one transverse dimension of the core orcladding of the first or second optical waveguide varies longitudinallyalong at least a portion of the optical junction region.